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UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


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•^te&uy  ^JL*^/** 
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ELEMENTS 


METEOROLOGY, 

WITH  QUESTIONS  FOR  EXAMINATION, 

DESIGNED  FOR  SCHOOLS   AND   ACADEMIES. 


BY  JOHN(BROCKLESBY,  A.M., 

of  MaUjwnatici  and  Neural  Philosophy  In  Trinity  College,  Hartforf. 


See  page  151. 

ILLUSTRATED   WITH   ENGRAVINGS. 
TENTH  EEVISED  AND  ENLARGED  EDITION. 

"  Fire  and  hail  ;  snow  and  vapor; 
Stormy  wind  fiilfllling  His  word," 

NEW    YORK: 
SHELDON  AND  COMPANY,  PUBLISHERS, 

498 


1869. 


Entered  according  to  Act  of  Congress,  in  the  year  1848,  by 

JOHN    BROCKLESBY, 
In  the  Clerk's  Office  of  the  District  Court  of  Connecticut. 


CASE,   LOCKWOOD   &  CO., 

KI.KtTKOTYPKKS    AM)    1MUXTKKS, 
IIAKTFORD,   CONN. 


•<>nr 


PREFACE. 


METEOROLOGY  is  a  subject  of  interest  to  all.  We  live 
in  the  very  midst  of  its  phenomena,  and  are  constantly 
subjected  to  their  influence.  Many  of  the  singular  pro- 
cesses of  nature  which  this  science  unfolds,  are  intimate- 
ly connected  with  our  being  and  happiness,  while  others, 
on  account  of  their  beauty  and  sublimity,  fill  the  mind 
with  admiration  and  awe. 

The  subject  being  one  of  universal  interest,  we  might 
naturally  suppose  it  to  be  universally  understood  ;  but 
such  is  not  the  case.  Meteorology,  as  a  science,  is  of 
recent  origin  ;  for  it  is  only  within  the  space  of  a  veiy 
few  years  that  it  has  risen,  through  the  efforts  of  many 
gifted  minds,  to  the  rank  it  deserves  to  hold  amid  the 
various  departments  of  knowledge. 

Meteorology  is  a  portion  of  Natural  Philosophy,  and 
in  the  colleges  of  our  land,  lectures  upon  this  subject 
form  a  part  of  the  regular  academical  course ;  but  no 
similar  system  prevails  in  our  High  Schools  and  Acade- 
mies. Nor  is  it  to  be  expected  ;  since,  with  the  present 
want  of  facilities  for  obtaining  information,  the  teacher 
would  be  obliged  to  devote  an  undue  share  of  his  time 
to  the  acquisition  of  the  knowledge  requisite  for  this 
object.  Neither  can  a  text-book  be  procured;  for  the 
author  is  not  aware  that  any  distinct  treatise  on  this 
science  is  extant  in  the  English  language,  except  the 

i50 


iV  PREFACE. 

translation  from  the  German  of  Kaemtz's  "  Complete 
Course  of  Meteorology ;"  a  work  which,  though  exceed- 
ingly valuable  to  the  advanced  student,  is  not  suitable 
for  a  text-book  on  account  of  its  size,  expense,  and  mode 
of  execution. 

The  present  little  work  has  therefore  been  prepared, 
not  with  the  view  of  adding  one  more  to  the  long  list  of 
studies  now  pursued  in  our  academical  institutions  ;  but 
for  the  purpose  of  bringing  into  general  notice  a  rich  but 
hitherto  comparatively  unknown  field,  within  the  do- 
mains of  natural  science. 

The  author  has  therefore  endeavored,  while  retaining 
all  the  important  principles  of  Meteorology,  to  condense 
the  subject  as  much  as  possible,  in  order  that  this  ele- 
mentary treatise  may  be  studied  in  connection  with  Nat- 
ural Philosophy,  without  consuming  too  much  time. 

In  regard  to  facts,  they  have  been  sought  wherever 
it  was  supposed  they  could  be  found,  and  reference  has 
been  made  in  nearly  all  cases  to  the  authorities  whence 
they  were  taken. 

Should  it  be  required  a  more  extended  treatise  may  be 
expected,  adapted  to  the  wants  of  students  in  colleges. 

HARTFORD,  July  7th,  1848. 


REFERENCES. 

(C.  957),  for  example,  denotes  Comstock's  Philosophy,  Article  957,  (last 

edition. 
(Art  132),  for  example,  denotes  Article  132  of  tlxis  work. 


TABLE    OF    CONTENTS. 


PREFACE 5 

Subject  defined 13 

PART   I. 

OF    THE     ATMOSPHERE. 

Barometer 14 

Temperature 18 

Capillarity 18 

Pressure  of  the  Atmosphere 19 

Variations  in  Latitude 19 

Variations  in  Altitude 21 

Density  of  the  Atmosphere 22 

Weight  of  the  Atmosphere 24 

Temperature  of  the  Atmosphere 24 

Thermometer 25 

Self-registering  Thermometer 27 

Mean  Daily  Temperature 29 

Variations  of  Temperature  in  Latitude 30 

Variations  of  Temperature  in  Altitude 30 

Humidity  of  the  Atmosphere 32 

Absolute  Humidity 33 

Relative  Humidity 33 

Hygrometer 34 

Pleight  of  the  Atmosphere 36 

PART   II. 

AERIAL     PHENOMENA. 

CHAPTER    I. 

OF   WINDS    IN    GENERAL. 

Cause  of  Wind 38 

Velocity 39 

Anemometer 40 

Force  of  Winds 40 

Trade  Winds 41 

Origin 41 

Limits  of  the  Trade  Winds 43 

Calms 44 

Winds  of  the  Higher  Latitudes 45 

Upper  Westerly  Wind  of  the  Tropics . .  48 

Periodical  Winds .  . 48 

Monsoons 48 

Origin 49 

Land  and  Sea  Breezes 50 

Origin 50 

Variable  Winds 51 

Physical  Nature  of  Winds 52 

Puna  Winds 52 

Simoom 52 

Sirocco 54 


vj  CONTENTS. 

CHAPTER    II. 

OF   HURRICANES.  Pan 

Hurricanes  defined  ...........................................   ,,. 

P;,ih  of  the  Storm  .............................................    ,R 

velocity  ................................................  ;;;;  g 

Diameter  ......................................  '  -7 

Examples  ...................................................  g 

Fall  of  the  Barometer  ........................................  ^ 

Sailing 


Axis  of  the  Hurricane  ........................................    '!. 

Espy's  Theory  .............................................. 

CHAPTER    III. 

OF  TORNADOES   OB   WHIRLWINDS. 

Facts  .......................................................  f 

Origin  ......................................................   f. 

Wi.irUvinds  excited  by  Fires  .................................  "5 

Results  of  Centrifugal  Action  .................................  V, 

Effects  of  Expansion  .........................................  "7 

CHAPTER    IV. 

OP  WATEB-SPOUTS. 

Water-spouts  defined  .........................................  &* 

Dimensions  .................................................   £• 

Popular  Error  ................................................    g| 

Sand  Pillars  .................................................    « 

Beneficial  Effects  of  Winds  ...................................  73 

PART  III. 

AQUEOUS    PHENOMENA. 

CHAPTER   I. 

OF    BAIW. 

Cause  of  Rain  ...............................................   71 

Piain  Gauge  .................................................   75 

Distribution  of  Rain  in  Latitude  ..............................   75 

•>ns  ..................................................   76 

I  Rain  ................................................   77 

Distribution  in  Altitude  ......................................   77 

Uaiii  upon  Coasts  ............................................   78 

Rains  within  the  Tropics  .....................................   7!) 

Seasons  ...............................................  7(.) 

......................................................  80 

India  .....................................   *0 

i  'al  Rains  ol  Congo  ....................................  81 

in  the  High*  r  Latitudes  ......  ..............  "82 

Winds  ................................................  82 

us  without  Rain  ........................................  83 

.......................................................  83 

......................................................  84 

:it  Rains  ..............................................  84 

i  ve  Showers  ...........................................  85 

.  ithout  Clouds  .........................................  85 

Cause  ........................  ..  86 


CONTENTS.  Vil 

CHAPTER    II. 

or  FOGS.  p^e 

Fogs  defined . .  bl? 

Constitution 81 

Distribution  in  Latitude 87 

Tropical  Regions 87 

Temperate  Regions 87 

Polar  Regions 87 

Cause 88 

Local  Distribution 88 

Rivers 8'J 

Mountains 90 

Capes 91 

Shoals 91 

Newfoundland 91 

England 91 

Garuas 92 

CHAPTER   III. 

OF  CLOUDS. 

Clouds  defined 94 

Strata  of  Clouds 95 

Thickness 96 

Height. 96 

Clouds  on  Mountains 97 

Classification 99 

Cirrus 99 

Cumulus 100 

Stratus 103 

Cirro-stratus 102 

Cirro-cumulus 103 

Cumulo-stratus 104 

Nimbus 105 

CHAPTER   IV. 

OP    DEW. 

Dew  defined 106 

Deposition 106 

Influence  of  Condition  of  the  Atmosphere 107 

Humidity 107 

Serenity 107 

Tranquillity 108 

Evening  and  Morning 108 

Influence  of  the  Substance  bedewed 109 

Constitution    109 

Surface  and  Form 109 

Location 110 

Color Ill 

Observations Ill 

Facts  Explained 112 

Beneficent  Distribution 112 

CHAPTER   V. 

OP   HOAR-FROST   AND    SNOV. 

Hoar-frost 113 

Snow 115 

1* 


,.:;:  CONTENTS. 

Page 

Snow  Flake J{jj 

Snow  Crystals *"! 

Natural  Snow  Balls *'j? 

Re<l  Snow .00 

Green  Snow J~o 

Cause ,.,. 

Uses  of  Snow 

CHAPTER  VI. 

OF  HAIL. 

Hail  J» 

Ii^u:r::::\":\"^:^:':'"":^"":v":::\:::\::::::l5| 

Geographical  Distribution !23 

Origin J*J 

Volta's  Theory g{ 

Olmsted's  Theory J** 

Curve  of  Perpetual  Congelation 1-^5 

Action  of  Opposite  Currents 126 

Action  of  Whirlwinds 128 

Influence  of  High  Mountains 129 

Hail  in  Southern  India 129 

PAKT   IV. 

ELECTRICAL,     PHENOMENA. 
CHAPTER    I. 

OF  ATMOSPHERIC   ELECTRICITY. 

Electrometers '. 131 

Electric  Condition  of  the  Atmosphere lc 

annual  Variation  in  Intensity 133 

Daily  Variation '.'....  133 

Variation  in  Altitude 134 

Origin 135 

Evaporation , 135 

Condensation 136 

Vegetation 136 

Combustion 136 

friction 137 

CHAPTER   II. 

OF  THUNDEB-STORMS. 

General  Distribution 137 

Origin 138 

Electrical  State  of  Thunder-clouds 139 

Electric  Action  of  Thunder-clouds 140 

Return  Stroke 140 

Height  of  Thunder-storms 142 

Lightning 143 

Origin 143 

Kinds 143 

lightning 144 

Sheet-lightning 144 

Ball-lightning 144 

Heat-lightning 145 

Velocity  of  Lightning 146 

color ;;;;  ;;  147 


CONTENTS.  IX 

Effects P$7 

Fulgurites 148 

Volcanic  Lightning 149 

Thunder 150 

Identity  of  Lightning  a.id  Electricity 150 

Franklin's  Experiment 151 

Romas'  Experiment 152 

Richman's  Death 152 

Lightning  Rod, 153 

Material 153 

Size 153 

Mode  of  Erection 153 

Extent  of  Protection 154 

Electric  Fogs 155 

Spontaneous  Electricity 156 

St.  Elmo's  Fire 156 

Electric  Rain,  Hail  and  Snow 157 

Electric  Action  upon  Telegraphic  Wires 158 

PART   V. 

OPTICAL     PHENOMENA. 

CHAPTER    I. 

OF  THE   COLOR   OP  THE    ATMOSPHERE   AND    CLOUDS. 

Color  of  the  Atmosphere 162 

Cyanometer 162 

Effect  of  Latitude 163 

Effect  of  Altitude 164 

Colors  of  Clouds 165 

CHAPTER    II. 

OF  THE   RAINBOW. 

Primary  Bow 170 

Secondary  Bow 172 

Breadth  of  the  Bows 173 

Position  and  Size  of  the  Rainbow 174 

Rainbows  in  the  North 175 

Extraordinary  Bows ]  75 

Supernumerary  Bows 176 

Lunar  Bows 176 

CHAPTER   III. 

OF  MIRAGE. 

Instances ^ 179 

Fata  Morgana 181 

Origin 183 

Erect  and  Inverted  Images  above  the  Object 183 

Magnified  Images 185 

Images  below  the  Object 187 

Images  produced  by  Reflection 188 

Spectre  of  the  Brocken 190 

Artificial  Mirage 191 

CHAPTER    IV. 

OF   CORONAS   AND    HALOES. 

Coronas 192 

Origin 193 


T  CONTENTS. 

Pagn 
Anthelia J)g 

:::::::::::::::::::::::::::::::::::::::::::::::::::::S 

Ordinary  Halo  of  45° ~01 

ulinary  Halo  of  90° 203 

-  passing  through  the  Sun 203 

Parhelia  and  Paraselenae 205 

PART    VI. 

LUMINOUS    PHENOMENA. 

CHAPTER   I. 

OF   METEORITES. 

Facts 206 

Meteorites 208 

Altitude 208 

Velocity 209 

Ac roli tes 209 

l\,:m 209 

Composition 210 

Meteoric  Iron 211 

Origin  of  Meteorites 212 

i  vpothesis 212 

Second  Hypothesis 213 

Third  Hypothesis 213 

Fourth  Hypothesis 214 

Filth  Hypothesis 216 

CHAPTER    II. 

OF   SHOOTING-STARS  AND   METEORIC   6HOWEHS. 

Altitude 216 

Velocity 217 

(':iiu>e 218 

Magnitude 218 

Splendor 219 

M'St'uiie  Showers 220 

.November  Epoch 220 

Varieties 221 

August  Epoch 222 

*. . . .  224 

Chaldni's  Hypothesis 225 

CHAPTER   III. 

OF   THE    AURORA    BOREALIS    OR    NORTHERN    LIGHT. 

ntion v 226 

Dark  Segment 226 

Arch  of  Light \\  228 

is 230 

230 

230 

Extent 232 

H'^'it 233 

Sounds  attending  the  Aurora 234 

Time '.  235 

Frequency 335 

Disturbance  of  the  Magnetic  Needle 237 

"  239 

;  ;;  240 


CONTENTS.  XI 

PART  VII. 

MISCELLANEOUS    PHENOMENA. 

CHAPTER    I. 

CV  THE  FALL  OP  TERRESTRIAL  SUBSTANCES  FOREIGN  TO  THE  ATMOSPHERE. 

#uet-storms  and  Blood-rains 24 1 

Dust-storms 242 

Instances 242 

Blood-rains ; 245 

Instances 245 

Black  Rain 247 

Red  Hail 248 

Black  Hail ..." !..'".".'  L  .'.."!.'.'.'.'!.'.'".'.'.'  248 

Storms  of  Colored  Snow 248 

Red  Snow 248 

Black  Snow 249 

Nature  of  the  Dust 249 

Infusoria 250 

Structure 250 

The  Italian  Dust-shower  of  1803,  and  the  Calabrian  of  1813 251  • 

Atlantic  and  Cape  de  Verd  Dust 251 

Sirocco  Dust 253 

Number  of  Distinct  Organisms  Discovered 256 

Number  and  Extent  of  Dust-storms  and  Blood-rains 256 

Their  Periodicity 257 

Origin  of  the  Dust 257 

Volcanic  Showers 259 

Cause 259 

Instances — Jorullo 259 

Souffriere 259 

Tomboro 260 

Cosiguina 261 

Yellow  Rains— Pollen-rains 262 

Gossamer-shower 262 

CHAPTER  II. 

DRT  FOG  AND  INDIAN  SUMMER  HAZE, 

Dry  Fog 264 

Instances 265 

Cause 266 

Indian-summer  Haze 266 

Cause 268 


METEOROLOGY. 


1.  METEOROLOGY,  is  THAT   BRANCH  OF  NATURAL 

SCIENCE    WHICH    TREATS    OF    THE    ATMOSPHERE   AND 

ITS  PHENOMENA.  The  subject  may  be  properly  divided 
into  six  parts. 

2.  PART  I.     THE   ATMOSPHERE. 

PART  II.  AEPJAL  PHENOMENA— comprehend- 
ing Winds  in  general,  Hurricanes,  Tornadoes,  and 
Water-spouts. 

PART  III.  AQUEOUS  PHENOMENA— including 
Rain,  Fogs,  Clouds,  Dew,  Hoar-frost  and  Snow,  and 
Hail. 

PART  IV.  ELECTRICAL  PHENOMENA— com- 
prising Atmospheric  Electricity  and  Thunder-storms. 

PART  V.  OPTICAL  PHENOMENA—  including 
the  Color  of  the  Atmosphere  and  Clouds,  Rainbow, 
Mirage,  Coronas,  and  Haloes. 

PART  VI.  LUMINOUS  PHENOMENA—  embra- 
cing Meteorites,  Shooting  Stars  and  Meteoric  Showers, 
z.nd  the  Aurora  Borealis. 

PART    VII.      MISCELLANEOUS    PHENOME- 

NA — including  the  Fall  of  Terrestrial  Substances 
foreign  to  the  Atmosphere,  and  Dry  Fog  and  Indian 
Summer  Haze. 

What  is  Meteorology  7 

Into  how  many  parts  is  it  divided? 

Rehearse  the  several  parts  with  their  subject*. 


PART    I. 


OF    THE    ATMOSPHERE. 

3.  As  the  common  properties  of  the  air,  viz.,  weight, 
finiility  and  elasticity,  are  supposed  to  be  already  known. 

C.  502,)  we  shall  proceed  at  once  to  the  discussion  of 
ihe  entire  body  of  air,  termed  the  atmosphere  ;  and  first 
of  its  pressure,  which  is  ascertained  by  the  barometer, 
an  instrument  so  called  from  the  Greek  words,  baros, 
weight,  and  metron,  measure. 

BAROMETER. 

4.  This  instrument  is  of  the  highest  importance  in 
Meteorology,  and  requires  a  minute  description.      It  is 
thus  constructed.     Into  a  glass  tube,  about  three  feet 
in  length,  open  at  one  end  and  closed  at  the  other,  mer 
cury  is  poured  until  it  is  full ;  the  open  end  being  no 
closed  by  the  finger,  or  any  other  means,  the  tube  is  i 
verted,  and  the  lower  end  immersed  in  a  vessel  of  me 
cury.     When  beneath  the  surface  of  the  fluid  the  end 
is  unstopped,  and  the  column  of  mercury  within  the  tube 
then  settles  down,  until  its  summit  is  about  thirty  inch- 
es above  the  level  of  that  within  the  vessel.     The  space 
above  the  column  in  the  tube  is  a  void,  and  is  called 
the  Torricellian  vacuum,  from  Torricelli,  the  name  of 
the  Italian  philosopher,  who  first  constructed  this  instru- 
ment.   • 

5.  The  column  of  mercury  within  the  tube  is  sup- 
ported above  the  level  of  that  in  the  vessel,  by  the  up- 
ward pressure  of  a  column  of  the  atmosphere,  having 
ihe  same  base  as  itself. 

What  is  the  atmosphere? 

How  is  its  pressure  ascertained? 

How  is  the  barometer  made  ? 

What  supports  the  column  of  mercury  1 


BAROMETER. 


15 


6.  Thus,    in    fig.    1.,    the    atmospheric          Fig- l- 
column  a  a,  of  indefinite  length,  but  of  the 

same  size  as  the  barometric  column  Db, 
presses  upon   the    mercury  in  the  vessel. 
K,  with  a  force  equal  to  its  own  weight ; 
now  since  any  force,  acting  upon  a  fluid, 
is  communicated   in   every  direction,  this' 
pressure  will  be  transmitted  through  the  < 
mercury,   in  the  direction   of  the- arrows,  * 
and  acts  at   D,   within  the  tube,  against 
the  mercurial  column  Db.     This  upward 
force  will  be  resisted  at  D,  by  the  weight 
of  Ub,  and  the  mercury  will  sink  in  the 
tube  until  the  two  pressures  counterpoise 
each  other,  in  exactly  the  same  manner 
as  two  equal  weights  in  the  opposite  scales 
of  a  balance. 

7.  From  these  considerations,  it  is  man- 
ifest, that  the  weight  of  the  atmospheric 
column  a  a  is  equal  to  that  of  the  mercuri- 
al column^  Db  of  the  same  base  ;  and  this 
weight  can  be  estimated  in  the  following 
manner.     If  the  base  at  D  contains  one 
square  inch,  the  column  Db,  at  its  usual 

height,  will  contain,  nearly,  30  cubic  inches  ;  and  since 
one  cubic  inch  of  mercury  weighs  3426.70  grains,  the 
weight  of  thirty  will  amount  to  102802.8  grains. 

This  product  being  now  divided  by  7004,  the  number 
of  grains  in  a  pound  avoirdupois,  the  result  will  be 
nearly  14.7  Ibs.  ;  a  quantity  equal  to  the  weight  of  the 
barometric  column,  and  consequently  to  the  pressure  of 
the  atmosphere  on  every  square  inch  of  surface. 

8.  Any  increase  in  the  density  of  the  atmosphere  will 
be  denoted  by  an  elevation  of  the  mercury,  and  a  de- 
crease by  its  depression.     The  cause  of  this  is  obvious, 
in  the  first  case,  a  a  becomes  heavier,  and  requires  more 


Explain  Figure  1. 

How  is  the  pressure  of  the  atmosphere,  on  every  square  inch,  computed  ? 
How  does  any  change  in  the  density  of  the  air  affect  the  height  of  the 
barometer  1 


16  THE    ATMOSPHERE. 

mercury  to  balance  it ;  therefore  Db  is  lengthened.  In 
the  second  case,  a  a  is  lighter,  and,  as  a  less  quantity  of 
mercury  will  then  balance  it,  Db  is  shortened.  Such 
changes  are  constantly  occurring,  but  are  very  minute, 
and,  in  order  that  they  may  be  accurately  indicated,  the 
instrument  must  be  made  with  the  nicest  care. 

9.  To  secure  a  perfect  instrument,  it  is  essential  that 
the  mercury  should  be  free  from  any  solid  impurities, 
else  the  summit  of  the  column  will  either  be  above,  or 
below,  its  proper  level,  according  as  the  foreign  matter, 
mixed  with  the  mercury,  is  lighter,  or  heavier,  than  the 
fluid.     This  end  is  attained  by  straining  the  mercury 
through  chamois  leather.      If  it  is  amalgamated  with 
zinc,  or  lead,  it  is  purified  by  washing  it  with  acetic,  or 
sulphuric  acid. 

10.  When  the  tube  is  filled,  moistufe  and  small  bub- 
bles of  air  are  found  adhering  to  its  interior  surface,  and 
are  also  contained  in  the  mercury.     These,  if  not  expell 
ed,  will  ascend  when  the  tube  is  inverted  into  the  Torri- 
cellian vacuum,  the  moisture  rising  in  vapor.     By  their 
united  elastic  force,  the  ascent  of  the  barometric  column 
will  be  checked,  whenever  any  increase  in  the  density 
of  the  atmosphere  tends  to  elevate  it. 

11.  This  source  of  error  is  removed  by  boiling  the 
mercury  in  the  tube.     When  all  the  air  and  vapor  are 
expelled,  the  tube,  if  gently  struck,  will  give  forth  a  dry, 
metallic  sound  ;  but  if  a  bubble  of  air  remains,  the  sound 
will  be  dull  and  heavy.     By  connecting  the  open  end  of 
the  tube  with  an  air  pump,  during  the  process  of  boiling, 
Dr.  Jackson,  of  Boston,  has  still  more  effectually  removed 
this  imperfection. 

12.  By  those  means,  the  air  may  perhaps  be  totally 
excluded,  when  the  instrument  is  first  constructed  ;  but 
in  the  course  of  time,  it  will  again  insinuate  itself  be- 
tween the  glass  and  the  mercurial  column.     To  pre- 
vent this  evil,  Prof.  Daniell,  of  King's  College,  London, 

\Yhru  precautions  are  adopted  to  secure  a  perfect  barometer  1 
How  is  the  mercury  purified,  and  why  1 
How  are  moisture  and  air  expelled  from  the  tube,  and  why? 
What  is  Prof.  Daniell's  improvement  1 


BAROMETER.  17 

welds  to  the  open  end  of  the  glass  tube  a  ring  of  plati- 
num, .which  possesses  a  greater  affinity  for  mercury  than 
glass.  The  mercury  adheres  closely  to  the  platinum, 
like  water,  and  the  passage  of  air,  according  to  all  ex- 
periments, appears  thus  to  be  effectually  prevented. 

13.  Since  the  constant  changes  in  the  weight  of  the 
atmosphere  produce  corresponding  fluctuations  in  the 
height  of  the  barometer,  a  scale  is  placed  near  the  top 
of  the  tube,  extending  from  twenty-seven  to  thirty-one 
inches,  a  space,  which  includes,  at  the  surface  of  the 
earth,  all  the  fluctuations  of  the  column.     This  scale  is 
divided  into  tenths  of  an  inch  ;  but,  as  the  variations  of 
the  barometer  are  exceedingly  minute,  a   contrivance, 
called  a  vernier,  is  annexed,  by  which  a  change,  to  the 
extent  of  one  five  hundredth  of  an  inch,  can  be  easily 
measured. 

14.  As  the  surface  of  the  mercury,  in  the  reservoir,  is 
raised  by  the  descent  of  the  column,  and  depressed  by 
its  elevation,  any  change  in  the  height  of  the  barometer 
cannot  be  accurately  estimated,  while  the  scale  remains 
in  the   same  position ;    unless  this    surface   is   always 
brought  to  the  same  point,  before  taking  an  observation. 
The  necessity  of  so  doing  will  be  obvious,  from  the  fol 
lowing  illustration. 

Suppose  the  surface  of  the  mercury  in  the  cistern  K, 
figure  1.,  to  be  fifty  square  inches,  while  that  of  a  hor- 
izontal section  of  the  column  is  but  one.  Should  the 
barometer  sink  one  inch,  the  surface  of  the  mercury  in 
the  cistern  will  rise  one  fiftieth  of  an  inch,  and  the 
amount  of  the  depression  of  the  column,  if  measured 
from  this  surface,  will  be  only  forty-nine  fiftieths  of  an 
inch,  instead  of  one  inch,  its  true  depression. 

15.  The  contrivance  employed  by  Fortin,  a  celebrat- 
ed French  artist,  to  remove  this   error,  consists  in  ad- 
justing to  the  cistern  K,  fig.  1.,  a  movable  bottom,  which 
can   be  elevated  or  depressed,  by  means  of  the  screw 


What  is  the  length  of  the  barometric  scale  1 

How  small  a  variation  in  height  can  be  measured  ? 

W  hat  is  Fortin's  contrivance,  and  for  what  purpose  adopted  ? 


IS  THE    ATMOSPHERE. 

P,  until  the  surface  of  the  mercury  shall  just  touch  the 
fixed  ivory  index  L,  at  its  lower  extremity  ;  which  point 
is  the  zero  of  the  scale,  or  the  place  from  which  the 
height  of  the  harometer  begins  to  be  reckoned. 

Id.  When,  by  adopting  the  previous  precautions,  the 
barometer  has  been  so  far  perfected,  two  corrections 
%re  still  necessary,  before  recording  observations ;  the 
irst  for  temperature,  and  the  second  for  capillarity. 
That  of  temperature  depends  upon  the  expansion  of 
-.he  mercury  and  the  scale ;  the  latter  being  partially 
corrective  of  the  former,  inasmuch  as  the  divisions  of 
measurement  upon  the  scale,  lengthen,  at  the  same 
time,  with  the  column  of  mercury. 

17.  TEMPERATURE.     Mercury  dilates,  for  every  de- 
gree Fah.  about  one  ten-thousandth  part  of  its  bulk, 
taken  at  the  freezing  point.     The  expansion  of  the  scale 
varies  with   the  metal  of  which  it  is  composed,  but  its 
amount  is.  usually,  so  small,  that  it  may  safely  be  neg- 
lected in  the  required  correction.     Hence  the  following 
practical  rule  has  been  adopted,  for  reducing  any  ob- 
served altitude  of  the  barometer,  to  the  corresponding 
altitude,  at  the  freezing  point.     "  Subtract  the  ten-thou- 
sandth part  of  the  observed  altitude,  for  every  degree  - 
above   the  freezing  point."      Thus,   if  the   barometer 
stands  at  29  inches,  and  the  thermometer  at  52°,  the 
required   correction   is   20  x  .0001  x  29  =  058.      If   the 
temperature  is  below  32°,  the  correction  must  be  added. 
To  facilitate  these  calculations,  a  thermometer  is  always 
attached  to  the  barometer. 

18.  CAPILLARITY.    By  capillary  attraction  is  (C.  53,) 
vnilrrxtnml.  the  force  exerted  by  the  interior  surface  of 
small  tubes,   ii/ion  the  fluids  contained  within  them. 
When  the  iluid  moistens  the  tube,  it  rises  above  its  pro- 
per level ;  but  when  it  does  not,  as  in  the  case  of  mer- 
cury, it  sinks  below  it.     From  this  cause,  a  depression, 
termed  its  capillarity,  occurs  in  the  barometer,  the  extent 

How  is  the  barometer  affected  by  a  change  in  temperature  1 
Give  the  rule  for  reducing  the  height  to  the  corresponding  height  at  the 
freezing  point. 
Why  is  capillarity  a  source  of  error  ? 


PRESSURE    OF    THE    ATMOSPHERE.  19 

of  which  is  dependent  upon  the  size  of  the  interior  diam- 
eter of  the  tube,  and  a  correction  for  this  must  be  added 
to  the  apparent  height,  in  order  to  obtain  the  true  alti- 
tude. In  tubes  of  a  small  bore,  the  error  from  this 
source  is  considerable  ;  but  when  the  diameter  exceeds 
half  an  inch,  it  becomes  so  small,  that  it  may  safely  be 
neglected.  This  will  be  rendered  evident  by  the  inspec- 
tion of  the  following  table,  which  gives  the  amount  of 
depression  for  tubes  of  various  sizes. 

Diameter  of  tube.  Depression. 

luoht-s.  inches. 

.10 1403 

.20 0581 

.40 0153 

.50 0083 

19.  When  the  instrument  is  not  stationary,  but  is 
carried  from   clime  to  clime,   and   to  different  heights 
above  the  sea-level,  two  other  corrections  are  necessary ; 
one  for  the  varying  force  of  gravity,  in  different  latitudes, 
and  the  other  for  the  change  of  pressure,  which  dimin- 
ishes with  every  increase  of  altitude  above  the  ocean. 

Such  is  the  barometer,  an  instrument  of  great  prac- 
tical use,  and  of  the  highest  value  in  meteorological  re- 
searches. 

PRESSURE  OF  THE  ATMOSPHERE. 

20.  VARIATION  IN  LATITUDE.  The  mean  or  average 
pressure  of  the  atmosphere,  as  indicated  by  the  barom- 
eter, is  found  to  be   nearly  the  same   in  all  latitudes, 
when  every  essential  correction   is  made.     It  increases 
a  little  from  the  equator  to  about  the  30th  degree  of 
latitude,  where  it  is  greatest  ;  it  then  decreases  to  nearly 
the  64th  degree,  where  it  is  least ;  after  tj^is  it  again 
increases,  and  between  the   75th  and  76th  degrees,  the 
•pressure  is  equal  to  that  of  the  equatorial  climes.    Ml 

Is  it  greater  in  tubes  of  a  small  or  large  bore  1 

When  the  barometer  is  portable,  what  other  corrections  are  necessary  ? 

What  is  said  of  the  barometer  1 

In  what  manner  does  the  pressure  of  the  atmosphere  vary  in  latitude  ? 


20 


THE    ATMOSPHERE. 


this  is  obvious  from  the  following  table,  founded  upon 
observations,  where  corrections  are  made  for  gravity, 
altitude  above  the  sea-level,  and  temperature. 


HEIGHT    OP 

Cape  of  Good  Hope,  . 
Christianburg,  .  .  . 
Tripoli  

33°  S. 

5°  30'  N. 
33°  N. 

Inches. 

29.955 
29.796 

30.127 

64°  N. 

29.593 

Spitsbergen,  .  .  . 

75°  30'  N. 

29.801      ' 

21.  The  pressure  of  the  atmosphere  at  any  given 
spot  is  not  invariable ;  for  the  height  of  the  barometer 
is  perpetually  changing  throughout  the  year.     The  ex- 
tent of  its  fluctuations    is,  however,  by   no  means  the 
same  in  all  places,  being  least  at  the  equator,  and  great- 
est towards  the  poles.    Thus  its  range.within  the  tropics 
is  but  a  little  more  than  one-fourth  of  an  inch  ;  at  New 
York,  40°  42'  40"  N.  Int.,  2.265  inches,  from  the  observa- 
tions of  five  years  ;  at  St.  Johns,  Newfoundland,  47°  34X  3" 
N.  lat.,  2.54  inches,  during  the  same  period ;  while  in 
Great  Britain  it  amounts  to  three  inches.     The  greatest 
fluctuations  occur  between  the  30th  and  60th  degrees  ot 
latitude. 

22.  There  is  also  a  constant  daily  variation  in  the 
atmospheric  pressure,  for  the  barometer,  as  a  general 
rule,  falls  from   10  o'clock,  A.  M.  to  4,  P.  M.  ;  it  then 
rises  until  10,  P.  M.,  when  it  again  begins  to  descend, 
reaching  ifs  lowest  point  at  4,  A.  M. ;  from  this  time  it 
rises,  until  it  once  more  attains  its  highest  elevation,  at 
10.  A.  M.     These  variations  are  exceedingly  minute, 
and  contrary  to  the  annual  range,  are  greatest  at  the 
cifiKilor,  and  ilrrrrttxr.  with  the  latitude  •  disappearing 
about  the  parallel  of  60°. 

23.  This  variation  amounts  at 


Give  examples. 

Where  are  the  annual  fluctuations  of  the  barometer  least? 

Where  greatest  1 

Give  examples. 

Describe  the  diurnal  variations.     Where  greatest  1    Where  least  ? 


PRESSURE    OF    THfi    ATMOSPHERE. 


21 


PLACES. 

LATITUDE. 

INCHES. 

Rio  Janeiro,      .    .     . 
Lima,  
Calcutta,  
St.  Petersburg,     .     . 

22°  54'  S. 
12°  3'    S. 
2»°  35'  N. 
59°  56'  N. 

to    .067 
to    .10 
to    .072 
to    .005 

In  the  tropical  regions,  according  to  Humboldt,  so  reg- 
ular are  the  diurnal  changes,  that  the  barometer  indi- 
cates true  time,  within  a  quarter  of  an  hour. 

These  daily  fluctuations,  in  the  atmospheric  pressure, 
for  a  long  time,  perplexed  meteorologists,  but  their  cause 
has,  at  length,  been  discovered,  by  means  of  the  late  ob- 
servations, at  the  English  observatories.  They  are 
found  to  arise  from  the  stated  variations  in  temperature, 
that  occur  during  the  day. 

24.  VARIATIONS  IN  ALTITUDE.     As  we  ascend  above 
the  surface  of  the  earth,  we  leave  a  portion  of  the  at- 
mosphere below  us,   and   are  freed  from  its   pressure. 
This  fact  is  denoted  by  the  fall  of  the  barometer.  When 
De  Luc,  a  French  philosopher,  ascended  to  the  height 
of  20,000  feet,  his  barometer  sunk  to  twelve  inches. '    In 
1838,  the  aeronaut  Green,  rose  from  Vauxhall  gardens,  in 
London,  to  an  ele.vation  of  nearly  three  miles  and  three 
quarters;  the  mercury  in  the  barometer  gradually  de- 
scending, from  thirty  inches  to  fourteen  and  seven-tenths. 

25.  As  a  general  rule,  this  depression,  near  the  sur- 
face of  the  earth  amounts  to  one-tenth  of  an  inch  for 
every  eighty-seven  feet  in  altitude  ;  but  where  perfect 
accuracy  is  required,  several  corrections  must  be  made. 
The  barometer  then  becomes,  in  the  hands  of  skillful 
observers,  an  important  instrument  for  determining  alti- 
tudes, and  so  exact  are  its  indications,  that  two  hide- 

Give  examples. 

What  is  said  by  Humboldt  of  their  regularity  in  the  tropics  1 

How  are  they  caused  ? 

How  is  the  pressure  of  the  air  influenced  by  the  altitude! 

What  instrument  indicates  the  changes  of  pressure  1 

In  the  instances  given,  how  low  did  the  mercury  sink  1 

What  is  the  law  of  depression  1 

For  what  purpose  is  the  barometer  sometimes  employed  1 

Give  instances. 


TITK    ATMOSPHERE. 


peuder  t  estimates  of  the  height  of  Mount  ^Etna,  made 
b\-  means  of  this  instrument,  differ  only  one  foot;  that 
of  ('apt.  .Smyth  being  10,874  feet,  while  Sir  John  Her 
schel's  is  10,873  feet. 

DENSITY  OF  THE  ATMOSPHERE. 

26.  When  one  portion  of  the  atmosphere  is  said  to 
be  more  dense  than  another,  all  that  is  meant  is  simply 
;his  ;  that  a  given  volume,  or  bulk,  of  the  first  portion, 
as  one  gallon,  contains  more  aerial  particles  than  an 
equal  volume  of  the  second  ;  thus,  if  it  contains  twice  as 
many  particles,  it  is  said  to  be  twice  as  dense. 

27.  The  density  of  the  atmosphere  decreases  with 
the  altitude.     This  result  is  caused  by  the  diminished 
pressure  of  the  air,  and  the  decreasing  force  of  gravity. 
Imagine  the  atmosphere  to  he  divided  into  a  vast  num- 
ber of  thin,  concen- 
tric strata,  which  in 

fiirure  2,  are  repre- 
sented by  the  spaces 
between  the  lines 
1-2,  2-3,  3-4,  4-5, 
&e. 

Now    it  is   clear, 
that  the  particles  in 

each  layer  are  pressed  together  by  the  whole  weight  of 
the  atmosphere  above  them,  while,  at  the  same  time, 
they  are  drawn  together  by  the  force  of  gravity.  Vari- 
ations in  the  latter  power  are  only  appreciable  at  great 
distances  from  the  earth,  and  the  observed  changes  in 
density,  at  t\\o  or  more  stations,  may  therefore  be  as- 
cribed to  the  dill'erence  in  the  weight  or  pressure,  of  the 
superincumbent  atmosphere.  The  height  of  the  barom- 
eter, at  different  elevations,  thus  denotes  the  density  of 
the  air  at  these  points. 


Fig.  2 


When  is  one  portion  of  the  atmosphere  denser  than  another  1 

Wli.-it  two  onuses  principally  influence  its  density  ?    Describe  figure  2. 

Which  cause  may  be  neglected  1 

What  instrument  measures  the  density! 


DENSITY    OF    THE    ATMOSPHERE.  23 

23.  The  density,  however,  is  not  exactly  proportioned 
to  the  pressure,  slight  modifications  arising,  from  sev- 
eral causes  ;  the  most  important  of  which  is  tempera- 
ture. The  heat  of  the  atmosphere  decreases  with  the 
altitude,  and  since  heat  expands,  and  cold  «on  tracts,  a 
given  volume  of  air,  rhjth  part  of  its  bulk,  at  32°,  for 
every  degree  Fah. ;  or  in  other  words,  thus  lessens  and 
increases  its  density,  a  correction  must  be  made  for  this 
influence. 

29.  It  has  been  found  by  calculation,  combined  with 
observations,  that,  if  the  altitudes  are  represented  by  an 
increasing  arithmetical  series,  the  densities  of  the  at- 
mosphere decrease  in  a  geometrical  progression.  Thus, 
if  at  the  height  of  18.000  feet  the  air,  as  the  barometer 
indicates,  is  but  half  as  dense,  as  at  the  surface  of  the 
earth  ;  at  36.000  feet  it  will  be  reduced  to  oA-fourth, 
and  at  54,000  feet  to  one-eighth  of  its  original  density. 

30.  The  rarefaction  of  the  air  at  lofty  elevations,  les- 
sens the  intensity   of  sound,  impedes  respiration,   and 
causes  the  minute  veins  of  the  body  to  swell  and  open. 
Thus,  at  a  short  distance,  the  report  of  a  pistol  upon  the 
summit  of  Mont  Blanc,  can  scarcely  be  heard.     Gay 
Lussac  and  Biot.  ascending  from  Paris,  in  a  balloon,  to 
the  height  of  25,000  feet,  breathed  with  pain  and  diffi- 
culty, and  upon  the  high  table  lands  of  Peru,  the  lips  of 
Dr.  Tschudi,  cracked  and  burst;  while  the  blood  flowed 
from  the  veins  of  his  eyelids. 

In  consequence  of  this  diminution  of  pressure,  water 
boils,  in  such  situations,  at  a  comparatively  low  tempe- 
rature ;  thus,  at  Quito  in  Equador,  9,537  feet  above 
the  sea  level,  ebullition  takes  place  at  the  temperature 
of  196°  Fah. 


In  what  manner  does  temperature  affect  the  density  1 

What  is  the  law  of  decrease  in  reference  to  altitude  ?    Illustrate. 

What  are  the  effects  of  a  rarefied  atmosphere  1    Give  instances. 


THE    ATMOSPHERE. 


WEIGHT  OF  THE  ATMOSPHERE. 

31.  We  have  seen,  that  a  column  of  mercury,  about 
thirty  inches  in  height,  weighs,  at  the  surface  of  the 
earth,  exactly  the  same  as  a  column  of  the  atmosphere, 
possessing  the  same  base.     If  then  the  globe  was  cov- 
ered with"  an  ocean  of  mercury,  thirty  inches  in  depth, 
the  latter  would  occupy  the  identical  base  that  the  at- 
mosphere does  now,  and  their  respective  weights  might 
be  regarded  as  equal. 

32.  Under  this  supposition,  the  diameter  of  the  earth 
would  be  increased  five  feet.     The  difference  then,  in 
cubic  feet,  between  the  solidity  of  the  earth,  and  that  of 
a  globe,  whose  diameter  is  five  feet  greater,  will  equal 
the  nuu^er  of  cubic  feet  in  the  sea  of  mercury.     This 
numbermultiplied  by  the  weight  of  a  cubic  foot  of  mer- 
cury, viz.  818,125  Ibs.,  will  equal  that  of  the  whole  mass, 
which  is  the  same  as  the  weight  of  the  atmosphere.  This 
calculation  has  been  made,  and  amounts  to  more  than 
five  thousand  billions  of  tons. 

TEMPERATURE  OF  THE  ATMOSPHERE. 

33.  The  entire  body  of  air  surrounding  tHe  globe  ap- 
pears to  be  warmed  in  two  ways  ;  first  by  the  luminous 
beams  of  the  sun,  secondly,  by  the  radiation  of  heat 
from  the  earth. 

34.  According    to    Kaemtz   and    Martin,  the   atmo- 
sphere absorbs  nearly  one-half  the  daily  amount  of  heat, 
which  emanates  from  the  sun  to  the  earth,  even  when 
the  sky  is  perfectly  serene.     The  remaining  portion  fall- 
ing upon  the  surface  of  the  ground,  elevates  its  tempera- 
ture, and  the  earth  sends  back  into  the  atmosphere  rays 
of  invisible  heat. 

35.  Modern  researches  have  shown,  that  all  bodies, 
through  which  heat  can  pass:  absorb  a  greater  propor- 

How  is  the  weight  of  the  atmosphere  computed  ? 
Kow  many  tons  does  it  weigh  1 
How  is  the  atmosphere  warmed  ? 


THERMOMETER.  25 

lion  of  non-luminous,  than  of  luminous  calorific  rays. 
The  heat,  therefore,  that  radiates  from  the  earth,  will 
not  pierce  the  atmosphere,  with  (he  power  of  the  solar 
ray  ;  all  will  be  retained  by  the  lower  strata  of  air, 
which  in  their  turn,  diffuse  invisible  thermic  rays,  ia 
every  direction. 

36.  We   thus   perceive,   what  all    observations  have 
proved,  that  the  upper  regions  of  the  atmosphere  must 
be  colder  than  the  lower.     It  is  not,  however,  to  be  for- 
gotten, that  the   rarefaction  of  the  superior  strata  coil- 
tributes  to  this  condition. 

THERMOMETER. 

37.  The  temperature  of  the  atmpsphere  is  indicated 
by  the  thermometer,  an  instrument,  which  derives  its 
name  from  the  Greek  words,  thermos,  warm,  and  metron, 
measure.     It  consists  of  a  small  glass  tube,  terminated 
by  a  bulb,  and  is  partially  filled  with  mercury. 

This  fluid  is  usually  preferred  for  several  reasons,  the 
most  important  of  which  are,  its  uniform  dilation,  its 
quick  susceptibility  to  any  change  in  temperature,  and 
the  great  range  of  its  expansion  in  the  fluid  state.  If 
the  instrument  is  to  be  exposed  to  extreme  cold,  alcohol 
must  be  used.  • 

38.  As  mercury,  like  other  fluids,  expands  by  heat, 
and  contracts  by  cold,  its  alternate  elevations  and  de- 
pressions within  the  tube,  can  be  made  to  indicate  the 
corresponding  changes  in    the  state  of  the  air,  if  two 
fixed  temperatures  can  be  found,  whence  to  Beckon  the 
changes.     These  have  been  discovered.     If  a  thermom- 
eter is  immersed,   at  different  times,  in  melting  snow, 
the  column  of  mercury  invariably   sinks  to  the  same 
place  in    the   tube,  though   many  months    may  have 
elapsed  between  the  experiments ;  and,  when  exposed 
to  the  steam  of  boiling  water,  the  mercury  always  as- 

1s  it  heated  most  by  luminous  or  non-luminous  heat'] 
Are  the  upper  or  Lower  regions  of  the  atmosphere  the  warmest  1 
How  is  the  temperature  of  the  atmosphere  measured  1 
Describe  the  thermometer.     Why  is  mercury  used? 
How  are  the  two  fixed  temperatures  obtained  ? 
9 


26 


THE    ATMOSPHERE. 


Fig.  3. 


.  Boiling  point 


cends  to  the  same  height,  under  the  same  atmospheric 
pressure. 

39.  These  invariable  positions,  which  are  termed  the 
freezing  and  the  boiling  points,  ate  marked  upon  the 
scale  to  which  the  tube  is  affixed.  In  Fahrenheit's  ther- 
mometer, figure  3.,  the  interval 
between  them  is  divided  into  180 
parts,  each  of  which  is  called  a 
degree  (1°)  and  as  the  freezing 
point  is  marked  32°,  the  boiling 
is  therefore  212°.  The  divisions 
are  extended  downwards  from 
32°  to  0,  or  the  zero  point,  and 
when  extreme  degrees  of  cold  are 
to  be  measured,  the.  range  is  con- 
tinued to  20°,  40°,  and  even  60° 
below  zero.  If  the  air  is  colder 
than  40°  below  zero,  a  spirit  ther- 
mometer must  be  used,  since  mer- 
cury becomes  solid  at  this  tem- 
perature. When  Simpson,  a  late 
northern  traveller,  wintered,  in 
1838,  at  Fort  Confidence,  67°  N. 
lat.,  he  cast  a  bullet  of  mercury, 
'.he  temperature  being  49°  below 

zero.  Upon  firing  the  ball,  it  passed  through  an  inch 
plank,  at  the  distance  of  ten  paces  ;  but  flattened  and 
broke  against  the  wall,  three  or  four  paces  beyond.  In 
addition  to  the  mode  of  graduation  adopted  by  Fahren- 
heit, several  others  prevail  (C.  570),  which  it  is  not  ne- 
cessary here  to  discuss. 

40.  The    thermometer   employed    for   meteorological 
purposes,  should  be  made  as  accurate  as  possible,  and  in 


Freezing  polni 
Zero. 


Into  how  many  intervals  is  the  space  between  them  divided  in  Fahrtn 
heit's  scale  1    What  are  the  intervals  called  1 
How  many  degrees  is  the  freezing  point  1 
How  many  the  boiling  point  1 
What  is  the  zero  point  1 
When  must  a  spirit  thermometer  be  used1? 
Relate  Simpson's  experiment. 


SELF-REGISTERING    THERMOMETER.  27 

order  to  ensure  its  perfection,  many  niceties  must  be  ob- 
served in  its  construction. 

41.  FIRST.     The    tube    should    be   of   equal    size 
throughout  the  whole  stem  ;  else  the  same  increase  of 
temperature  will  not  produce  the  same  increase  in  the 
height  of  the  mercury,  throughout  every  part  of  the 
tube  ;  and  so  of  the  decrease. 

SECONDLY.  The  bulb  should  be  large  in  proportion 
to  the  tube  ;  for  then  slight  changes  in  temperature  will 
be  rendered  perceptible,  and  the  delicacy  of  the  instru- 
ment increased. 

THIRDLY.  The  mercury  should  be  pure,  dry,  and 
recently  boiled,  in  order  to  free  it  from  air ;  and,  when 
in  the  tube,  should  there  again  be  boiled,  to  drive  off 
any  air  or  moisture  collected  within. 

LASTLY.  When  the  mercury  is  at  the  summit  of  the 
tube,  and  every  thing  else  has  been  expelled,  the  top  of 
the  tube  'must  be  perfectly  closed  by  the  fusion  of  the 
glass,  leaving,  when  the  mercury  has  cooled,  a  void 
space  or  vacuum  above. 

42.  When  a  thermometer  has  been  exposed  to  great 
changes  in  temperature  during   the   course  of  a  year, 
the  position  of  the   freezing   point  upon    the  scale  is 
found  to  be  somewhat  altered  ;   for,  if  the  instrument  is 
then  placed  in  melting  snow,  the   mercury  is  usually 
seen  to  stand  a  little  higher  than  32°,  and  less  than  33°. 
This  change  would  occasion  a  constant  error  in  the  ob- 
servations,  and    meteorologists    therefore   verify   their 
thermometers  at  stated  intervals,  in  the  way  just  men- 
tioned, 

^      SELF-REGISTERING  THERMOMETER. 

43.  The   object   for  which    this  instrument   is  con- 
structed, is  to  obtain,  in  the  absence  of  the  observer,  the 
highest  and  lowest  temperature  of  the  day,  or  of  any 
other  interval  of  time. 

What  precautions  must  be  taken  to  construct  an  accurate  thermometer  1 

What  change  occurs  in  the  position  of  the  freezing  point  1 

How  are  thermometers  verified  1 

For  what  purpose  is  a  self-registering  thermometer  used  1 


28 


THE    ATMOSPHERE. 


One  of  the  most  correct  thermom- 
eters of  this  kind,  now  in  use,  is  that 
invented  by  Mr.  James  Six,  of  Col- 
chester, which  is  represented  in  Fig.  4. 

It  consists  of  a  long  glass  bulb,  G, 
narrowing  into  a  fine  tube,  which  is 
first  bent  downward,  forming  the 
arm  a  b,  and  then  upwards,  forming 
the  arm  c  d,  which  terminates  in  a 
small  cavity,  L.  The  two  arms  con- 
tain mercury,  which  extends  down 
from  a  on  one  side,  and  up  to  c  on 
the  other  :  the  bulb  and  the  rest  of 
the  tube  are  filled  with  alcohol,  ex- 
cept the  upper  part  of  the  cavity  L. 
Upon  the  top  of  the  mercury  in  each 
arm  rests  an  index  (which  is  more 
perfectly  seen  at  A),  consisting  of  a 
piece  of  iron  wire  capped  with  ena- 
mel, and  loosely  twined  with  a  fine 
glass  thread  ;  when  the  mercury  de- 
scends, the  index  would  fall,  were  it 
not  for  the  glass  thread,  which,  press- 
ing like  a  spring  against  the  sides  of 
the  tube,  supports  the  index,  in  any 


44.  The  action  of  the  instrument  THERMOMETER. 
is  as  follows  :  When  an  increase  of  temperature  ex- 
pands the  spirit,  the  mercury  is  depressed  in  the  arm 
a  6,  and  elevated  in  c  d,  carrying  the  index  up  with  it. 
If  the  temperature  now  falls,  the  spirit  contracts,  and 
the  mercury  descends  in  c  d]  but  the  index  remains  in 
its  last  position,  from  the  pressure  of  the  glass  spring 
against  the  tube  ;  and,  as  it  does  not  fit  tightly  to  the 
latter,  the  alcohol  above  it  flows  readily  by. 

As  the  cold  augments,  the  mercury  rises  in  a  b,  bear- 
ing up  the  index  of  this  arm,  until  an  increase  of  tem- 
perature occurs,  when  the  mercury  here  falls,  and  the 


Describe  Six's,  from  fig.  4. 


MEAN    DAILY   TEMPERATURE.  29 

index  continues  stationary.  Thus,  the  highest  point  co 
which  the  index  rises  in  the  arm,  a  b,  indicates  the  least 
temperature,  and  that  in  cd  the  greatest,  that  happens 
in  any  interval  of  time,  as  a  day,  or  a  year ;  and  the 
scale,  as  is  evident  from  the  figure,  is  graduated  accord- 
ingly. 

45.  After  every  observation,  each  index  requires  to 
be  adjusted  ;  this  is  done  by  means  of  a  magnet,  which, 
being  moved  down  the  side  of  the  arm,  draws  the  index 
after  it. 

Another  instrument  of  this  kind  was  invented  by 
Rutherford,  (C.  575.) 

MEAN  DAILY  TEMPERATURE. 

46.  The  mean  or  average  temperature  of  the  day, 
would  be  accurately  found   by  observing  the  thermom- 
eter at  intervals  of  an  hour  during   the  whole  twenty- 
four,  and  dividing  the  sum  of  the  temperatures  by  the 
number  of  observations,  viz.,  twenty-four.    This  method 
is  however  too  laborious,  and   meteorologists  have  en- 
deavored to  arrive  at  the  same  result  from  two  or  three 
daily  observations. 

47.  According  to  Kaemtz,  a  celebrated  German  me- 
teorologist, if,  in  Germany,  the  thermometer  is  noted  at 
6,  A.  M.,  2,  P.  M.,  and  10,  P.  M.,  and  the  sum  of  the 
temperatures  divided  by  three,  the  quotient  will  differ 
but  little  from  the  true  mean.     The  rule  adopted  in  the 
State  of  New  York,  under  the  direction  of  the  Regents 
of  the  University,  is  as  follows  : 

Mark  the  temperature,  first,  between  daylight  and 
sunrise  ;  secondly,  between  2  and  4,  P.  M.  ;  thirdly,  an 
hour  after  sunset :  add  together  the  first  observation, 
twice  the  second  and  third,  and  the  first  of  the  next 
day,  and  divide  the  sum  by  six  ;  the  result  will  be  the 
mean. 

The  mean  daily  temperature  at  Philadelphia  has 
been  found,  from  the  hourly  observations  of  Capt.  Mor- 

What  is  understood  by  the  mean  daily  temperature  ? 
How  is  it  obtained? 


30 


THE    ATMOSPHERE. 


decai,  to  be  one  degree  less  than  the  temperature  at 
9,  A.  M. 

48.  By  taking  the  average  of  all  the  mean  daily  tem- 
peratures throughout  the  year,  the  mean  annual  tem- 
perature is  ascertained.     It  is  also  obtained  by  the  aid 
of  the  self-registering  thermometer,  the  average  of  the 
t\vo  extreme  temperatures  being  regarded  as  the  mean 
of  each  day. 

49.  VARIATIONS  OF  TEMPERATURE  IN  LATITUDE. 
By  comparing  situations  differing  widely  in  latitude,  it 
is  found  that  the  average  annual  temperature  of  the 
atmosphere  diminishes  from  the  equator  towards  cither 
pole.     This  will  be  seen  from  the  annexed  table,  which 
presents  the  results  at  the  sea  level,  for  nine  places. 


PLACES. 

LAT. 

TEMP. 

PLACES. 

LAT. 

TEMP. 

Falkland  Isles, 
Buenos  Ay  res, 
Rio  Janeiro,  . 
Maranham,    . 
Trincomalee, 

51°  S. 
34°  36'  S. 
2-2°  56'  S. 
2°  29'  S. 
8°34'N. 

l^n 

62°  .6 
73°  .96 
81°  .32 
81°  .32 

Calcutta,   .    . 
Savannah,      . 
London,    .     . 
Mt-lville  Isle,  . 

22°  35'  IV. 
32°05'N. 
51°31'N. 
?4°47'N. 

Pahren. 
73°  .44 
64°  .58 
50°  .72 
1.66  be- 
low zero. 

50.  From  this  table  it  is  also  evident,  that  places  hav- 
ing the  same  latitudes,  in  the  two  hemispheres,  do  not 
necessarily   possess    the    same    average     temperature. 
This  is  owing  to  a  great  variety  of  local  causes,  the 
effect  of  which  cannot  always  be  accurately  estimated. 

51.  VARIATIONS  IN  ALTITUDE.     The  temperature  of 
the  air  diminixh.cs  with  the  altitude,  but  the  law  of  de- 
crease is  very  irregular,  being  affected  by  the  latitude, 
seasons,  hours  of  the  day,  and  a  diversity  of  local  cir- 
cumstances.    Jt  may  however  be  assumed,  as  a  gen- 
eral rule,  that  a  loss  of  heat  occurs  to  the  extent  of  one 
degree  Pah.  for  every  343  feet. of  elevation.    This  is  an 

How  is  the  mean  annual  temperature  found  1 

How  does  the  temperature  of  the  atmosphere  vary  in  respect  to  "latitude  1 

Give  examples 

Do  like  latitudes  in  different  hemispheres  have  the  same  temperature? 

How  is  the  temperature  affected  by  altitude  1 

What  is  the  general  law  of  decrease  1 


MEAN    DAILY    TEMPERATURE. 


31 


average  result,  for  the  rate  of  decrease  is  very  rapid  near 
the  earth,  after  which  it  proceeds  more  slowly,  but  at  the 
loftiest  heights  is  again  accelerated. 

52.  During  the  winter  of  1838,  the  French  scientific 
commission  stationed  at  Bossekop,  in  West  Finmark, 
09°  58  N.  lat..  found  this  law  partially  reversed,  amid 
the  rigors  of  a  polar  clime  ;    the    temperature  of  the 
atmosphere  increasing,  nearly,  3°  Fah.  for  the  first  328 
feet  in  height ;  beyond  this  limit  it  began  to  decrease,  at 
first  slowly,  but  afterwards  with  greater  rapidity.     Dur- 
ing the  summer,  the  temperature  decreased  with  the 
altitude. 

53.  As  a  consequence  of  this  gradual  reduction  of 
heat,  a  point  at  length  may  be  attained,  in  any  latitude, 
if  we  continue  to  ascend,  where  moisture,  once  frozen, 
always  remains   congealed.      Hence,  arise  the  eternal 
snows  and  glaciers,  that  crown  the  summits  of  the  high- 
est mountains. 

54.  Since  the  mean  temperature  of  the  air  is  highest 
at  the  equator,  and  sinks  towards  either  pole,  the  points 
of  perpetual  congelation  are  farthest  removed  above  the 
ocean-level  within   the  torrid  zone,  and  gradually  ap- 
proach nearer  the  general  surface  of  the  earth,  with  th 
increase  of  latitude  ;  as  the  following  table  shows. 


PLACES. 

LATITUDE. 

LOWER    LIMIT    OF 
PERPETUAL   SNOW. 

Straits  of  Magellan.  .     . 
Chili  

54°  S. 
41° 

3,706  feet. 
6  009     '• 

Quito,      
Mexico,    

00° 

19°  N 

15,807     " 
14  763     « 

JEtna,       

37°  30' 

9  531     " 

Kamschatka,    
Isle  of  Mageroe,  Norway,' 

56°  40' 
71°  15' 

5,243     " 
2,362     « 

55.  A  striking  departure  from  the  rule  exists,  how 
ever,  in  India ;  for  while  on  the  south  side  of  the  Him- 
•vnalehs,  the  snow  line  occurs  at  the  heigjit  qf  about 

Was  it  found  true  at  Bossekop  ? 

What  results  from  this  gradual  loss  of  heat  7 

Where  are  the  points  of  perpetual  congelation  nearest  to  the  ocean? 

Where  farthest  from  it  1    Give  examples. 


32  THE    ATMOSPHERE. 

13,000  feet,  on  the  northern  acclivity  it  rises  to  the  alti- 
tude of  17,000.  Many  explanations  of  this  singular  lacl 
have  been  given,  which  admit  not  of  discussion  here. 

HUMIDITY  OF  THE  ATMOSPHERE. 

56.  At  all  temperatures  moisture  resides  in  the  atmo- 
sphere, self- sustained,  in  an  invisible  state.    Between  the 
particles  of  air  intervals  are  believed  to  exist,  which  are, 
either  partially,  or  wholly,  filled  with  the  vapor  that 
constantly  rises  from  the  earth. 

57.  This  peculiarity  in  the  constitution  of  the  atmo- 
sphere is  termed  Ike  capacity  of  the  air  for  'moisture, 
and  when  the  intervals  are  full  of  vapor,  it  is  said  to  be 
suln rated.     An  increase  of  temperature,  by  dilating  the 
air,  separates  the  particles  farther  from  each  other ;  the 
intervals  are  thus  enlarged,  and  the  capacity  of  the  air 
incrvtised.     A  diminution  of  temperature  is  followed  by 
contrary  effects;  the  size  of  the  intervals  is  then  redti 
oed.  and  the  capacity  lessened. 

58.  The  capacity  increases,  however,  at  a  faster  tale 
:  hem  the  temperature.     A  volume  of  air,  at  32°  Fan.  is 
capable  of  containing  a  quantity  of  moisture,  equal  to  the 
':•  (JOih  part  of  its  own  weight ;  but  for  every  twenty  seven 
additional  degrees  of  heat,  this  quantity  is  doubled. 

Tims  a  body  of  air  can  contain, 

At    32°  Fah.  the  IGOth  part  of  its  own  weight. 
"    59°  "      80th     "  " 

"    86°  "      40ih     "  " 

"  113°  "      20th     «  " 

From  this  it  follows,  that  while  the  temperature  ad- 
vances in  an  arithmetical  series,  the  capacity  is  accel- 
erated in  a  geometrical  progression. 

What  departure  from  this  rule  exists  ? 

What  docs  the  atmosphere  contain  at  all  temperatures'? 

What  is  mea^t  by  the  capacity  of  the  air  for  moisture  1 

When  is' the  air  said  to  be  saturated? 

What  is  the  effect  of  heat  upon  the  capacity  1 

What  \3  the  effect  of  cold? 

Which  increases  at  the  fastest  rate,  temperature  or  capacity  1 

Give  instances.    What  is  the  rule  in  respect  to  temperature  and  capacity  1 


HUMIDITY    OF    THE    ATMOSPHERE,  33 

59.  ABSOLUTE  HUMIDITY.    From  the  cause  just  men- 
tioned, it  would  naturally  be  inferred,  that  the  quantity 
of  atmospheric  vapor,  or  the  absolute  humidity,  is  great- 
est in  the  equinoctial  regions,  and  diminishes  towards 
either  pole  ;  a  conclusion  abundantly  supported  hy  facts 
as  will  be  shown  hereafter. 

60.  The  air  over  the  ocean  is  always  saturated,  and 
upon  the  coasts,  in  equal  latitudes,  contains  the  greatest 
possible  amount  of  vapor  ;  but  the  quantity  decreases  as 
we  advance  inland,  for  the  atmosphere  of  the  plains  of 
Oronoco,  the  steppes  of  Siberia,  and  the  interior  of  New 
Holland,  is  naturally  dry. 

61.  The  absolute  humidity  diminishes  with  the  alti- 
tude, but  the  rate  of  reduction  is  not  fully  known.     By 
comparing  different  seasons  and  hours,  it  is  found  to  be 
greater  in  summer  than- in  winter,  and  less  in  the  morn- 
ing than  at  about  mid-day. 

1  62.  RELATIVE  HUMIDITY.  This  must  not  be  con- 
lounded  with  absolute  humidity.  By  relative  humidity 
is  understood  the  dampness  of  the  atmosphere,  or  its 
proximity  to  saturation  ;  a  state  dependent  upon  the 
mutual  influence  of  its  absolute  humidity  and  tempera- 
ture ;  for  a  given  volume  of  air  may  be  made  to  pass 
from  a  state  of  dampness,  to  one  of  extreme  dry  ness,  by 
merely  elevating  its  temperature,  without  altering,  in  the 
least,  the  amount  of  moisture  it  contains. 

Thus  one  hundred  and  sixty  grains  of  air,  containing 
one  grain  of  vapor,  would  be  damp  at  36°  Fah.,  but  hot 
and  withering  at  the  temperature  of  90°.  By  the  revet se 
of  this  process,  a  body  of  hot  air  will  not  only  become 
humid,  but  will  even  part  with  a  portion  of  its  original 
moisture,  if  it  is  cooled  down  to  any  great  extent. 

63.  From  the  numerous  observations  of  Kaemtz,  at 
Halle,  and  on  the  shores  of  the  Baltic,  it  appears  that 

What  Is  absolute  humidity  7    Where  is  absolute  humidity  the  greatest  7 
How  does  it  diminish  ?    Where  is  the  air  always  saturated  1 
What  is  said  of  inland  regions?    What  is  the  effect  of  altitude? 
Compare  summer  and  winter,  morning  and  mid-day. 
What  is  relative  humidity?    Upon  what  does  it  depend?    Illustrate  the 
effects  of  a  change  of  temperature,  the  absolute  humidity  being  the  same. 
2* 


34  THE    ATMOSPHERE. 

the  relative  humidity,  in  those  situations,  is  highest  in 
the  morning  before  sunrise,  and  lowest,  or  farthest  re- 
moved from  the  point  of  saturation,  at  the  hour  of  the 
greatest  diurnal  heat.  Corresponding  results  have  been 
obtained  in  this  country. 

HYGROMETER. 

64.  Those  instruments  by  which  the  humidity  of  the 
atmosphere   is  measured  are  called  hygrometers,  from 
the  Greek  words  ugros,  moist,  and  metron,  measure.  Of 
these  there  exists  a  great  variety,  differing  both  in  form 
and  piinciple  ;  but  those  are  esteemed  the  most  accurate 
in  their  indications,  that  are  constructed  upon  the  prin- 
ciple of  condensation,  to  which  allusion   has  already 
oeen  made,  (Art.  62.)  but  a  more  extended  explanation 
is  here  required. 

65.  Imagine  a  brightly  polished  metallic  vessel,  par- 
tially filled  with  water,  at  the  temperature  of  60°  Fah., 
to  be  placed  in  a  room  at  the  same  temperature.     If 
pieces  of  ice  are  now  thrown  into  the  vessel,  the  water 
is  gradually  cooled  down,  and  as  this  reduction  proceeds, 
the  lustre  of  the  exterior  surface  will  be  dimmed,  at  a 
certain  moment,  by  a  fine  dew.     This  is  caused  by  the 
deposition  of  moisture  from  the  atmosphere,  which,  in 
contact  with  the  cold  surface  of  the  vessel,  is  now  cool- 
ed down  just  beyond  the  point  of  saturation.     The  tem- 
perature of  the  water  at  this  instant,  which  is  the  same 
as  that  of  the  vessel,  is  termed  the  dew-point. 

66.  By  marking  the  difference,  in  degrees,  between 
the  temperature  of  the  air  and  the  dew-point,  the  rela- 
tive dryness  of  the  atmosphere,  or  its  remoteness  from 
saturation   is  obtained.      But  observations,  like  these, 
lead  also  to  other  important  results;  for,  by  the  aid  of 
tables,  giving  the  elastic  force  of  aqueous  vapor,  at  dif 
ferent  temperatures,  the  absolute  weight  of  the  vapor, 
diffused  through  a  given  volume  of  air  can  be  determin- 

Wha  did  Kaemtz  observe  in  respect  to  relative  humidity? 
What  is  a  hygrometer?    Explain  the  principle  of  condensation. 
What  is  the  dew-point  1    How  is  the  relative  humidity  obtained  1 
What  other  results  can  be  deduced? 


HUMIDITY    OF    THE    ATMOSPHERE. 


35 


ed,  and  likewise  the  proportion  it  contains,  to  that  which 
would  be  required  to  saturate  it. 

67.  The  hygrometer  of  Prof. 
Daniell,    which    is    extensively 
used,  is  thus  constructed. 

A  glass  tube,  e  i,  figure  5.,  is 
bent  twice  at  right  angles,  and 
terminated  by  two  bulbs,  b  and/, 
of  the  same  material.  The  bulb 
b  is  partly  filled  with  ether,  into 
which  is  inserted  the  ball  of  a 
delicate  thermometer,  d,  enclosed 
in  one  arm  of  the  instrument. 
All  air  is  excluded  from  the  tube, 
which  is  filled  with  the  vapor  of 
ether ;  the  other  bulb, /,  is  cov- 
ered with  a  piece  of  fine  mus- 
lin, a,  and  upon  the  pillar,  g  h, 
a  second  thermometer,  k  I,  is  fixed. 

68.  Observations  are  thus  made.  The  instrument  being1 
placed  by  an  open  window,  or  out  of  doors,  a  few  drops 
of  good  ether  are  suffered  to  fall  upon  the  muslin-covered 
bulb,  which,  from  the  rapid  evaporation  of  the  ether, 
soon  becomes  cool,  condensing  the  ethereal  vapor  with- 
in.   In  consequence  of  this  effect,  the  ether  in  b  evapo- 
rates, thus  causing,  not  only  in  the  ether,  but  also  in  the 
enclosing   bulb,  a  reduction  of  temperature,  which  is 
measured  by  the  interior  thermometer,  e  d. 

As  the  evaporation  at  a  proceeds,  the  temperature  of 
b  still  continues  to  fall,  and,  at  a  certain  point,  the  at- 
mospheric vapor  will  be  seen  gathering  in  a  ring  of  dew 
upon  the  glass,  and  the  difference  in  degrees,  at  this 
moment,  between  the  external  and  internal  thermome- 
ter, denotes  the  relative  dryness  of  the  atmosphere. 
Thus,  if  on  one  day  the  exterior  thermometer  stood  at 
65°,  and  the  enclosed  sunk  to  50°  ere  the  dew-ring  ap- 
peared— and  on  another,  the  former  was  at  73°,  and  the 
latter  had  descended  to  68°  before  the  glass  was  dimmed 


Describe  Darnell's  hygrometer,  fig.  5.,  and  explain  the  mode  of  taking 
observations. 


36  THE    ATMOSPHERE. 

with  moisture— in  the  first  instance  the  dryness  of  the 
Mmi'sphere  would  be  indicated  by  15°,  and  in  the  second 
by  5. 

()'.).  The  action  of  this  instrument  is  almost  instan- 
taneous, for  the  enclosed  thermometer  begins  to  fall  in 
two  seconds  after  the  ether  is  dropped.  It  is  usual, 
where  great  precision  is  required,  to  read  off  the  de- 
grees of  the  interior  thermometer  at  the  moment  the 
dew-ring  appears,  and  also  at  the  moment  it  vanishes  ; 
the  average  of  the  two  observations  being  taken  as  the 
true  dew-point. 

70.  In  England    the  dew-point  is  seldom  30°  Fall, 
below  the  temperature  of  the  air ;   the  greatest  differ- 
ence at  Hudson,  Ohio,  as  given  by  Prof.  Loomis,  is  36°. 
In  the  tropical  regions  its  range  is  the  most  extensive  ; 
for,  in  the   burning  clime  of  India,  the  dew-point  has 
sometimes  sunk  as  low  as  29°,  while  the  temperature 
of  the  atmosphere  was  90° — a  difference  of  sixty-one 
degrees. 

71.  HEIGHT  OF  THE  ATMOSPHERE.    Whether  the 
atmosphere  is  boundless  or   not,  is  a  question  which 
natural  philosophers   have   been   unable    to  determine. 
l)e  Luc  regards  it  as  unlimited,  and  imagines  the  plan- 
t't.'iry  spaces  to  be  filled  with  a  medium  so  exceedingly 
attenuated  as  not  to  retard  the  motions  of  the  heavenly 
orbs.     The  earth  and  the  various  celestial  bodies  are 
supposed  to  condense  this  subtil  fluid  around  them  into 
an  atmosphere,  by  virtue  of  their  respective  attractions. 

72.  Were  this  true,  the  densities  of  the  atmospheres 
thus  formed  would  differ,  on  account  of  the  variations  ir 
the  size  and  mass  of  these  bodies.     It  therefore  consti- 
tutes a  strong  objection  to  this  hypothesis,  that  the  den- 
sity of  the  atmosphere  of  Jupiter  (as  shown  by  the  re- 
fraction of  the  light  of  his  satellites,  at  the  period  of  their 
eclipses)  is  not  superior  to  that  of  our  own  ;  although 
the  force  of  attraction  at  the  surface  of  this  planet  is  al- 


IIow  far  below  the  temperature  of  the  air  does  the  de\v-pjint  descend  in 
England  1  in  Ohio  1  in  India  1    Is  the  height  of  the  atmosj.nere  known  1 
What  is  De  Luc's  opinion  ?    What  is  th"  "^ipction  to  ths  hypothesis  "3 


HEIGHT    OF    THE    ATMOSPHERE.  37 

most  three  times  greater  than  that  of  the  earth.  More- 
over, when  Venus  passes  near  the  sun,  she  exhibits  no 
atmosphere,  according  to  Wollaston,  notwithstanding 
her  size  is  nearly  equal  to  that  of  the  earth. 

73.  Those  who  maintain  that  the  atmosohere  is  lim- 
ited, suppose,  that  at  a  certain  distance  from  the  earth, 
the  expansive  energy  of  its  particles  is  exactly  balanced 
by  the  force  of  gravity,  and  that  beyond  this  point,  an 
infinite  void  extends.     This  distance  has  been  computed 
to  be  not  far  from  22,200  miles  from  the  centre  of  the 
globe. 

74.  Whichever  theory  may  be  adopted,  it  is  certain 
that  the  atmosphere  extends  to  very  great  heights.    Dr. 
Wollaston  has  shown,  by  calculation,  that  the  atmos- 
phere, at  the  altitude  of  nearly  forty  miles,  is  still  suffi- 
ciently dense  to  reflect  the  rays  of  the  sun,  when  this 
luminary  is  below  the  horizon.     It  is  capable  of  trans- 
mitting sound  at  a  loftier  elevation,  for  in  1783,  a  vast 
meteoric  body  exploded    at  an  altitude  of  more  than 
fifty  miles,  the  sound  reaching  the  earth  like  the  report 
of  a  cannon.     Still  farther  ;  if  the  combustion  of  meteors 
is  truly  assigned  to  the  action  of  the  atmosphere,  the  ex- 
istence of  the  latter,  at  the  distance  of  one  hundred  miles 
from  the  earth,  may  be  regarded  as  proved. 

What  do  the  advocates  of  a  limited  atmosphere  suppose  1 

How  far  is  this  point  from  the  earth's  centre? 

At  what  height  docs  the  atmosphere  reflect  light  1 

At  v/hat  altitude  transmit  sound  ? 

Wnat  inference  is  drawn  from  the  combustion  of  meteors  1 


390550 


PART   II. 

AERIAL    PHENOMENA. 

• 

CHAPTER  I. 

OF  WINDS   IN   GENERAL. 

75.  CAUSE  OF  WIND.  Wi?id  is  air  in  fnotion,  occur- 
ring whenever  the  repose  of  the  atmosphere  is  broken^ 
from  any  cause  whatsoever.  It  is  usually  the  result  of 
a  change  of  temperature,  and  consequently  of  density, 
but  the  rush  of  an  avalanche,  causing  a  sudden  displace- 
ment of  a  vast  volume  of  air,  has  been  known  to  pro- 
duce a  momentary  wind  of  great  violence,  along  the 
borders  of  its  path. 

70.  If  two  contiguous,  upright  columns  of  air,  with 
their  bases  at  the  same  level,  are  unequally  heated,  the 
colder  is  the  denser,  and  at  its  base  a  current  will  flow 
towards  the  lighter  column,  (just  as  the  compressed  air 
within  a  bellows  streams  out  into  the  rarer  atmosphere,) 
but  at  the  top,  to  supply  this  loss,  a  counter  current  pre- 
vails. * 

77.  This  is  illustrated  by  Franklin's  simple  experi- 
ment; if  a  door  is  opened,  communicating  between  a 
warm  and  cold  room,  and  a  lighted  taper  then  placed  at 
the  bottom  of  the  doorway,  the  flame  is  bent  towards  the 
warm  apartment ;  but  if  held  at  the  top,  its  direction  is 
reversed. 

78.  On  account  of  the  unequal  distribution  of  heat 

What  does  part  second  treat  of  1    What  does  chapter  first  treat  of '{ 
Define  wind.    When  does  it  occur  1 

If  two  contiguous  columns  of  air  are  unequally  heated,  what  motion 
takes  place  ]  State  Franklin's  experiment. 


OP    WINDS    IN    GENERAL. 


39 


over  the  surface  of  the  globe,  phenomena  like  these  occur 
in  nature,  on  a  widely  extended  scale  ;  for  if  two  neigh- 
boring countries  are  unequally  heated,  the  air  above 
them  partakes  of  their  respective  temperatures,  and 
there  arises  at  the  surface  of  the  earth,  a  wind  blowing 
from  the  colder  to  the  warmer  region,  while  at  the  same 
time,  a  directly  contrary  current  prevails  in  the  upper 
strata  of  the  atmosphere. 

79.  VELOCITY.  Every  gradation  exists  in  the  speed 
of  winds,  from  the  mildest  zephyr,  that  scarcely  bends 
the  flower,  to  the  most  violent  hurricane,  which  pros- 
trates the  giant  oak,  and  hurls  to  the  ground  the  proud- 
est works  of  man.  They  have  been  classed  as  follows, 
by  Smeaton,  according  to  their  rapidity  and  force. 


Velocity  of  the  wind, 
miles  per  hour. 

Perpendicular  force  on  one  square 
loot  in  Ibs.  avoirdupois. 

Common  appellation  of 
auch  winds. 

1 

.005 

Hardly  perceptible. 

4 
5 

.079 
.123 

£  Gentle  wind. 

10 
15 

.492 
1.107 

?  Pleasant  brisk  gale. 

20 
25 

1.063 
3.075 

1  Very  brisk. 

30 
35 

4.429 
6.027 

|  High  wind. 

40 

7.873 

Very  high. 

50 

12.300 

Storm. 

60 

17.715 

Great  Storm. 

80 

31.490 

Hurricane. 

100 

49.200 

Violent  Hurricane. 

80.  The  velocity  of  the  upper  currents  of  the  atmos- 
phere, is  as  variable  as  that  of  the  winds  which  sweep 
over  the  surface  of  the  globe  ;  for  the  aeronaut,  Green, 
who  ascended  from  Liverpool,  in  1839,  to  the  height  of 
14,000  feet,  encountered  a  current  that  bore  him  along 
at  the  rate  of  five  miles  per  hour,  but  upon  descending 
to  the  altitude  of  12,000  feet,  he  met  with  a  contrary 
wind,  blowing  with  a  velocity  of  eighty  miles  per  hour. 

How  does  it  explain  the  origin  of  winds  7 

What  is  said  of  the  velocity  of  winds  1 

Give  the  common  appellations  of  winds,  with  their  velocity  and  forc«. 

What  is  said  of  the  speed  of  the  upper  currents  1 

Give  examples. 


40  AERIAL    PHENOMENA. 

On  one  occasion,  his  balloon  was  carried  over  the  space 
of  ninety-seven  miles  in  fifty-eight  minutes. 

81.  ANEMOMETER.     The  velocity  of  the  wind  is  esti- 
mated by  the  anemometer,  an  instrument  so  called  from 
the  Greek  words,  ancmos,  wind,  and  metron,  measure. 
One  of  the  best  is  Woltmann's.     It  consists  of  nothing 
more  than  a  small  windmill,  to  which  is  attached  an 
index,  in  order  to  mark  the  number  of  revolutions  per 
minute;  the  number  of  course  increasing  with  the  speed 
of  the  wind.     Now  if  the  atmosphere  is  still,  and  the 
anemometer  is  carried  against  it  at  the  rate,  for  instance, 
of  ten   miles  per  hour,  the  number  of  its  revolutions 
will  be  exactly  the  same  as  if  the  instrument  was  sta- 
tionary. and  die  vanes  resolved  by  the  force  of  a  breeze 
possessing  the  same  velocity. 

82.  If  then,  upon  a  calm  day,  the   anemometer  is 
taken  upon  a  railroad  car.  moving,  for  example,  at  the 
speed  of  twenty  miles  an  hour,  and  the  number  of  revo- 
.utions  for  half  an  hour  accurately  noted,  we  can  obtain, 
(by  dividing  this  result  by  30,)  the  number  of  revolu- 
tions per  minute,  corresponding  to  those  of  a  wind  hav- 
ing a  velocity  of  twenty  miles  per  hour.    In  this  manner, 
a  table  adapted  to  the  instrument  can   be  constructed 
for  all  winds,  moving  with  a  greater  or  less  rapidity. 

The  velocity  of  the  higher  aerial  currents  is  ascer- 
tained by  the  speed  with  which  the  shadow  of  a  cloud 
passes  over  the  surface  of  the  earth. 

83.  FORCE.     The  force  of  the  wind  is  obtained,  by 
observing  the  amount  of  pressure  it  exerts  upon  a  given, 
plane  surface,  perpendicular  to  its  own  directions.     If 
the   pressure  plate  acts  freely  upon   spiral  springs,  the 
power  of  the  wind  is  denoted  by  the  extent  of  their 
compression,  and  that  weight  will  be  a  measure  of  its 
force,  which  produces  the  same  effect  upon  the  springs. 

This  instrument,  which  is  also  termed  an  anemometer, 


What  is  an  anemometer  ?    Describe  Woltmann's,  and  the  mode  of  com- 
puting by  it  the  velocity  of  the  wind. 
How  do  we  judge  of  the  speed  of  the  upper  currents? 
In  what  manner  is  the  force  of  the  wind  estimated  7 


TRADE    WINDS.  41 

is  constructed  in  exactly  the  same  manner  as  a  letter 
weigher,  where  a  weight  of  half  an  once  compresses  the 
spiral,  bringing  down  the  index  to  a  certain  division  of 
the  scale. 

84.  If,  however,  the  velocities  of  the  different  winds 
are  already  known,  and  the  force  of  one  obtained,  those 
of  the  rest  can  be  found  by  the  following  rule,  viz.  that 
their  forces  are  as  the  squares  of  their  velocities.     For 
instance,  if  the  power  of  a  gale,  possessing  the  speed  of 
twenty  miles  an  hour,  is  known  to  be  1,968  pounds  on  a 
square  foot,  that  of  a  storm  with  a  velocity  of  fifty  miles 
can  thus  be  ascertained  by  a  simple  proportion. 
(20x20)     (50x50)         Ibs.  Ibs. 

400  is  to  2500  as  1,968  is  to  the  answer  12,30. 
Should  the  forces  be  known,  it  is  obvious  that  the 
velocities  can  be  computed  by  reversing  this  process. 

Winds  may  be  divided  into  three  classes,  CONSTANT, 
PKRIODICAL,  and  VARIABLE. 

CONSTANT    WINDS.     TRADE    WINDS. 

85.  The  most  remarkable  instance  of  the  first  class, 
is  that  vast  current,  which,  in  the  torrid  zone,  is  ever 
sweeping  around  the  globe,  in  a  westerly  direction  ;  and, 
from  its  advantage  to  commerce,  in  always  affording  a 
steady  gale  to  the  bark  of  the  adventurous  mariner,  is 
denominated  the  trade  wind. 

86.  So  uniform  is  its  motion,  that  on  the  voyage  from 
the  Canaries  to  Cumana,  on  the  northern  coast  of  South 
America,  it  is  scarcely  necessary  to  touch  a  sail ;   and 
with  equal  facility,   the  richly  laden  Spanish  galleons 
were  accustomed  to  cross  the  Pacific  from  Acapulco  to 
the  Philippine  Isles. 

87.  ORIGIN.     The  cause  of  this  wind  has  been  thus 
explained  by  Halley,  an  English  philosopher.    From  the 
vertical  position  of  the  sun,  the  regions  near  the  equator 

If  the  velocities  are  known  and  one  force,  what  else  can  be  obtained  1 

Give  the  rule  and  the  example. 

If  the  forces  are  known,  what  can  be  computed  1 

Into  how  many  classes  are  winds  divided  1    Name  them. 

What  is  the  trade  wind  1    How  does  it  originate  1 


42  AERIAL    PHENOMENA. 

are  intensely  heated,  while  those  more  remotely  situated 
are  less  so  ;"  the  temperature  gradually  diminishing  to- 
wards either  pole.  (Art.  49.)  In  accordance  with  the 
principles  just  unfolded,  (Art.  78,)  an  upper  current  will 
flow  from  the  equator  towards  the  poles,  and  a  cold  cur- 
rent at  the  surface  of  the  earth,  from  the  poles  and  the 
higher  latitudes,  towards  the  equator.  Here  the  air, 
becoming  rarefied  by  the  heat,  rises,  and  mingling  with 
the  upper  wind  flows  back  again  to  the  polar  climes; 
thus  establishing  a  perpetual  circuit.  If  then  the  at- 
mosphere was  subject  to  no  other  influences,  a  north 
irlnd  would  prevail  in  the  torrid  zone,  in  the  northern 
hemisphere,  and  a  south  in  the  southern;  but  these 
directions  are  modified  by  the  rotation  of  the  earth,  in 
the  following  manner. 

88.  Every  thing  upon  the  surface  of  the  globe  at  the 
equator,  is  carried  towards  the  east,  at  the  rate  of  about 
sixty-nine  miles  in  four  minutes;  but  as  we  recede  to 
the  north  or  south  of  this  line,  the  eastern  velocity  is  so 
diminished,  that  at  the  latitude  of  60°  it  is  reduced  to 
one-half,  and  at  83°  to  less  than  one-eighth  of  its  original 
amount. 

A  wind,  therefore,  blowing  from  the  high  latitudes 
towards  the  equinoctial  clime,  is  constantly  passing  into 
regions  where  all  terrestrial  objects  have  a  greater  east- 
erly velocity  than  itself.  They  will  consequently  move 
a  gainst  it,  and  as  they  are  apparently  stationary,  it  will 
thus  acquire  a  relative  westerly  motion.  Just  as  when 
a  traveler,  outstripping  the  wind  that  blows  at  his  back, 
feels  a  breeze  directly  in  his  face. 

89.  Thus,  the  polar  wind  in  the  northern  hemisphere 
is  influenced  by  two  forces  at  the  same  time,  one  of 
which  carries  it  to  the  south,  and  the  other  to  the  west  • 
nud  the  course  it  assumes  by  their  combined  action  must 
he  according  to  the  laws  of  compound  motion,  (C.  249,) 
some  intermediate   direction,  tending  from  the  north- 
ec-st  to  the  south-west ;  and  such  is  the  fact,  according 
to  all  observations. 

What  two  forces  influence  the  polar  wind  in  the  northern  hemisphere  1 
What  is  the  direction  of  the  trade  wind  in  this  hemisphere? 


TRADE    WINDS.  43 

In  a  similar  manner,  the  lower  current  in  the  south- 
ern hemisphere,  acquires  a  direction  from  the  south-cast 
to  the  north-west. 

The  passage  of  a  vessel  across  a  river  is  an  illustra- 
tion in  poijgt.  If  the  vessel  is  steered  before  the  wind, 
from  east  to  west,  while  the  stream  is  flowing  from  north 
to  south,  she  will  be  seen  by  a  spectator  on  shore  sail- 
ing from  north-east  to  south-west. 

90.  In  the  Atlantic  and  Pacific,  the  breadth  of  the 
trades  increases  as  they  flow  towards  the  western  shores 
of  these  vast  oceans,  the  wind  gradually  changing  to 
the  east,  by  the  mutual  action  of  the  two  currents. 

91.  The  land  is  heated  by  the  sun  far  more  intensely 
than  the  ocean.    This  is  owing  to  the  fact  that  the  solar 
rays  warm  only  the  surface  of  the  earth,  scarcely  pene- 
trating an  inch  in  the  course  of  a  day,  while  during  the 
same  time  they  pierce  the  water  to  the  depth  of  many 
fathoms.     It  has  been  computed  that  the  beams  of  the 
sun  communicate  daily  a  hundred  times  more  heat  to 
a  given  extent  of  ground  than .  to  an  equal  surface  of 
water.    On  this  account,  the  proximity  of  highly  heated 
continents  produces  local  variations  in  the  direction  of 
these  winds ;  for  the  air,  being  more  rarefied  over  the 
land,  ascends,  and  to  supply  its  place,  the  cooler  air  of 
the  trades  sets  in  from  the  sea  towards  these  localities. 

92.  Thus,  on  the  African  coast,  between  Cape  Baja- 
dor  and  Cape  Verde,  a  north-west  wind  prevails  within 
the  limits  of  the  north-east  trade  ;  and  off  the  coast,  from 
Sierra  Leone  to  the  Isle  of  St.  Stephen,  the  trade  wind 
gradually  changes  to  the  south  and  south-west,  veering 
to  the  west  as  it  approaches  the  shore.     From  the  same 
cause,  the  south-east  trade  becomes  a  south  wind  along 
the  coasts  of  Chili  and  Peru. 

93.  LIMITS  OF  THE  TRADE  WINDS.     In  the  Pacific, 
the  north-east   trade  wind   prevails  between  the  25th 

What  in  the  southern  hemisphere'?    Illustrate  the  subject. 
What  is  said  of  the  breadth  of  the  trades  1 

Why  is  the  land  more  intensely  heated  than  the  ocean  7    How  does  this 
difference  cause  a  local  variation  in  the  direction  of  the  trades'? 
Give  instances  of  such  changes.    State  the  limit  of  the  trade  winds. 


44  AERIAL    PHENOMENA. 

and  2d  degree  of  north  latitude.  The  extent  of  the 
south-east  trade  is  not  precisely  ascertained,  but  it  prob- 
ably ranges  from  the  10th  to  the  21st  degree  of  south 
latitude.  In  the  Atlantic,  the  former  is  comprised  be- 
tween the  30th  arid  8th  degrees  of  north  latitude,  and 
the  latter  within  the  limits  of  the  3d  degree  ef  north 
and  the  28th  degree  of  south  latitude. 

94.  The  limits,  however,  are  not  stationary,  but  are 
dependent   upon    the  season — advancing   towards    the 
north  during  the  summer  of  the  northern  hemisphere, 
and  receding  to  the  south  as  the  sun  withdraws  to  the 
southern  tropic.     Thus,  on  the  west  coast  of  Europe,  the 
north-east  trade  has  been  found  to  extend  as  far  as  Ma- 
deira, and  even  to  Mafra,  in  Portugal. 

95.  CALMS.     In  the  vicinity  of  the  Cape  Verde  isles, 
between  the  8th  and  3d  degree  of  north  latitude,  is  a 
tract  denominated   by  mariners  the  rainy  sea.     This 
region  is  doomed  to  continual  calms,  broken  up  only  by 
terrific  storms  of  thunder  and  lightning,  accompanied 
by  torrents  of  rain.     A  suffocating   heat  prevails,  and 
the  torpid  atmosphere  is  disturbed,  at  intervals,  by  short 
and  sudden  gusts,  of  little  extent  and   power,   which 
blow  from  every  quarter  of  the  heavens,  in  the  space  of 
an  hour — each  dying  away  ere  it  is  succeeded  by  an- 
other.    In  these  latitudes,  vessels  have  sometimes  been 
detained  for  weeks. 

In  the  Pacific,  the  region  of  calms  is  comprised  within 
the  2d  degree  of  north  and  south  latitude,  near  Cape 
Francis  and  the  Galapagos  islands — a  narrow  belt  of 
ocean  separating  the  two  trades.  Here,  likewise,  dread- 
ful tempests  prevail. 

96.  According  to  Humboldt,  a  similar  state  of  the  at- 
mosphere exists,  during  the  months  of  February  and 
.'March,   on   the  western  coast  of  Mexico,  between  the 
13th  and  15th  degrees  of  north  latitude,  and  103d  and 
10()th  degrees  of  west  longitude.     A  ship,  richly  laden 
with  cocoa,  was  here  becalmed  for  the  space  of  twenty- 
Are  the  limits  stationary  1    Upon  what  do  they  depend  7 

Give  examples.    Where  is  the  rainy  sea?    Describe  it. 
Where  are  the  calms  in  the  Pacific  1    What  instance  is  given  1 


WINDS    OF    THE    HIGHER    LATITUDES.  45 

eight  days,  when  the  water  failing,  the  crew  were  com- 
pelled by  their  sufferings  to  abandon  the  vessel  and 
seek  the  shore,  eighty  leagues  distant,  in  an  open  boat. 

97.  The  calms  are  supposed  to  be  caused  in  the  fol- 
lowing manner.     The  adjacent  continents  to  the  east 
of  these   stagnant   regions   being   far   more   intensely 
heated  than  the  sea,  the  air  over  the  latter  would  rush 
easterly  towards  the  land,  were  it  not  arrested  by  a  con- 
trary impulse  in  the  direction  of  the  trade  wind.     If 
these  opposing  forces  are  at  any  time  equally  strong,  the 
atmosphere  is  motionless,   and  a  dead  calm  ensues — 
just  as  a  vessel,  in  ascending  a  stream,  continues  sta- 
tionary when  the  power  of  the  wind  is  exactly  balanced 
by  that  of  the  current.     When,  however,  the   relative 
strength  of  these  forces  rapidly  changes,  those  short  and 
sudden  gusts  which  have  been  noticed  will  arise,  as  one 
or  the  other  of  these  impulses  prevails. 

98.  The  presence  of  a  highly  heated  region  is  strik- 
ingly marked  in  the  case  of  the  rainy  sea.     To  the  east 
lies  the  great  African  desert,  from  whose  burning  surface 
a  vast  volume  of  hot  and  rarefied  air  is  perpetually  ris- 
ing. 

Another  cause  must  not  be  forgotten,  which  applies, 
more  particularly,  to  the  calms  near  Cape  Francis.  This 
tract  is  directly  under  the  equator,  and  from  its  peculiar 
situation,  the  upward  current  of  rarefied  air  is  probably 
here  so  strong  as  to  neutralize  the  action  of  the  trade 
winds. 

The  limits  of  the  calms  vary  also  with  the  seasons. 
Thus,  in  the  Atlantic,  the  range  in  August  is  between 
3°  15  and  13°  N.  Lat.,  but  in  February,  extends  from 
1°  15  to  6°  N.  Lat. 

WINDS    OF    THE    HIGHER   LATITUDES. 

99.  The   upper   equatorial  currents,  flowing  off  to- 
wards  either   pole,  descend,   on  their   passage,   to  the 
earth,  and  since  they  carry  with  them  an  excess  of  east- 
erly velocity,  will  become,  upon  the  principles  already 

How  do  the  calms  originate  ?    What  are  their  limits? 
What  is  the  direction  of  the  wind  in  the  higher  latitudes  ? 


46  AERIAL    PHENOMENA. 

explained,  (Art.  89,)  south-westerly  winds,  in  the  north- 
ern hemisphere,  and  north-westerly  in  the  southern. 

Such  would  be  the  course  of  these  currents  if  left  to 
themselves;  but  as  they  meet  on  their  passage  with 
counteracting  winds,  and  are  influenced  by  a  variety  of 
causes,  their  direction  is  more  or  less  changed  ;  yet  not 
so  much,  but  that  a  marked  predominance  in  the  fre- 
quency of  westerly  winds  exists  in  both  hemispheres. 

100   That  this  is  true,  in  regard  to  the  northern  hem- 


North 


•Russia.  N.  67'  W. 

West.  __ 

France,  S.  88 D  W 
North  America,  S.  86°  W. 

Germany,  S.  76°  W. 

England,  S.  66°  W.  „ 
Denmark,  S.  62°  W. 

Sweden,  S.  50°  W. 


South 

PREVAILING   DIRECTION   OP  THE   WIND   IN   DIFFERENT   COUNTRIES. 

*  Kaomtz  remarks  of  Russia,  "  that  the  number  of  observations  haw 
not  been  sufficient  to  enable  us  to  deduce  any  thing  conclusive." 

What  gives  them  this  direction  1 

Explain  fig.  6.,  and  give  the  course  of  the  wind  for  the  several  countries. 


WINDS    OP    THE    HIGHER    LATITUDES.  47 

isphere.  is  obvious  from  the  annexed  cut.  figure  6.,  which 
presents  the  results  of  a  multitude  of  observations.  A 
quarter  of  the  circumference  of  a  circle  is  here  supposed 
to  be  divided  into  ninety  parts,  called  degrees,  and  the 
inclination  of  the  several  lines  on  which  the  airows  are 
placed,  to  the  north  and  south  line,  measures,  in  degrees, 
the  average  or  mean  course  of  the  wind,  in  the  several 
places  mentioned. 

The  degrees  are  reckoned  from  the  south,  in  all  cases 
except  Russia,  where  they  are  counted  from  the  north. 
The  points  of  the  arrows  indicate  the  quarter  towards 
which  the  wind  blows. 

101.  The  prevalence  of  westerly  winds  in  the  high 
latitudes  of  the  north  is  also  shown  by  the  fact,  that  the 
average  length  of  the  outward  passage,  by  packet,  from 
New  York  to  Liverpool,  is  but  twenty-three  days,  while 
that  of  the  return  voyage  is  forty.     It  also  appears, 
from  the  observations  of  Hamilton,  during  twenty-six 
voyages  between  Philadelphia  and  Liverpool,  extending 
from  1798  to  1817,  that,  out  of  2029  days  on  which  the 
wind  blew,  it  came  from  the  west  1101  ;   a  result  agree- 
ing with  the  observations  of  McCord,  at  Montreal,  who 
found  that,  from  1836  to  1840,  inclusive,  the  westerly 
winds  at  this  station  constituted  more  than  one-half  of 
all  the  winds  that  blew,  bearing  the  ratio  of  54  to  100. 

102.  In  the  high  southern  latitudes,  the  same  fact 
has   been   observed.      Lieut.  Maury  remarks,  that   at 
Cape  Horn  there  are  three  times  as  many  westerly  as 
easterly  winds,  and  that  he  has  seen  vessels  arrive  at 
Valparaiso  and  Callao,  after  having  been  detained  off 
the  Cape,  by  gales  and   head  winds,  for  the  space  of 
eighty,  and  even  one  hundred  and  twenty  days.    In  the 
late  Exploring  Expedition,  the  ship  Vincennes  remained 
at  Orange  Harbor,  in  Terra  del  Fuego,  for  the  space  of 
sixty  days,  during  which  time  the  weather  was  exceed- 
ing variable  ;  for  thirteen  days  the  wind  blew  from  the 
north,  eastward,  and  south-east,  while  for  forty-seven,  it 
prevailed  from  the  west. 

State  facts  respecting  westerly  winds  in  the  high  northern  latitudes. 
The  same  in  regard  to  the  high  southern  latitudes. 


43  AERIAL    PHENOMENA. 

UPPER  WESTERLY  WIND  OF  THE  TROPICS. 

103.  The  prevalence  of  a  westerly  wind,  above  the 
trade;!,  within  the  torrid  zone,  is  shown  by  many  con 
elusive  facts.     In  1812,  ashes  from  the  volcano  of  St. 
Vincents  were  carried  easterly,  falling  upon  the  island 
of  Barbadoes;  and  the  captain  of  a  Bristol  ship  declared, 
that  at  this  time  volcanic  dust  descended  to  the  depth  of 
five  inches,  upon  the  deck  of  his  vessel,  at  the  distance 
of  five  hundred  miles  to  the  east  of  the  former  island. 

In  1835,  an  eruption  occurred  of  the  volcano  of  Con 
sanguma,  situated  in  Guatimala.  The  height  of  the 
crater  is  3800  feet,  and  from  it  issued  clouds  of  ashes, 
which  obscured  the  sun  for  five  days,  and  being  borne 
along  in  a  north-easterly  direction,  by  the  upper  cur- 
rent, fell  in  the  streets  of  Kingston,  Jamaica,  seven  hun- 
dred and  thirty  miles  distant.  Even  in  the  latitude  of 
Teneriffe,  nearly  all  travelers  have  found  a  westerly 
wind  at  the  summit  of  the  peak,  while  the  regular  trade 
was  blowing  in  a  contrary  direction,  at  the  level  of  the 
ocean. 

PERIODICAL   WINDS. 

104.  MONSOONS.     In    certain   countries  within   and 
near  the  tropics,  the  regular  action  of  the  trade  wind  is 
destroyed  by  the  monsoons,  which  are  periodical  gales, 
deriving  their  name  from  the  Malay  word  moussin,  sig- 
nifying  seasons.      These  winds   blow,  from  a  certain 
quarter,  for  one  half  of  the  year,  and  during  the  other 
half  from  an  opposite  point ;   and  at  the  time  of  their 
shifting,  dead  calms,  tempests,  and  variable  winds  alter 
nately  occur. 

105.  From  April  to  October,  the  south-west  monsoon 
prevails  north  of  the  equator,  and  the  south-east  in  the 
southern  hemisphere  ;    but  from  October  to  April,  the 
nortli-u'cxt  monsoon  blows  south  of  the  equator,  and  the 
north-east  in  the  northern  hemisphere. 

What  is  the  direction  of  the  wind  above  the  trades  1     Give  the  proofs. 
What  are  the  monsoons'?     In  what  manner  do  they  blow  1 
From  April  to  October  wltat  monsoons  prevail,  and  where  1 
From  October  to  April  what  monsoons  prevail,  and  where? 


PERIODICAL    WINDS.  49 

This  may  be  taken  as  a  general  rule,  subject  to  the 
following  modification  ;  that  the  south-west  and  north- 
west monsoons  occur  later  in  the  season,  according  as 
the  regions  over  which  they  prevail  are  farther  removed 
from  the  equator. 

Thus,  in  India,  at  Anjengo,  on  the  Malabar  coast. 
8°  30'  N.  Lat.,  the  south-west  monsoon  commences  as 
early  as  the  8th  of  April ;  at  Bombay,  19°  N.  Lat., 
about  the  15th  of  May.  In  Arabia,  it  begins  a  month 
later  than  on  the  shores  of  Africa,  and  in  the  northern 
part  of  Ceylon,  fifteen  or  twenty  days  earlier  than  on 
the  Coromandel  coast> 

106.  ORIGIN.     The  cause  of  these  regular  changes  is 
to  be  sought  in  the  effect  produced  by  the  sun,  during 
his   apparent  annual  progress  from   one  tropic  to  the 
other.     In  the  Indian  ocean,  for  example,  as  this  lumi- 
nary advances  towards  the  north,  the  zone  of  greatest 
rarefaction   recedes  from  the  equator,  and  the  north- 
east monsoon  (which  is   nothing  more  than  the  trade 
wind)  then  subsides,  and  is  succeeded  by  calms  and  va- 
riable winds  ;  but  as  the  summer  approaches,  and  the 
sun  arrives  at  the  northern  tropic,  the  southern  portions 
of  the  Asiatic  continent  become  hotter  than  the  ocean, 
and  the  humid  air  from  the  equatorial  seas  flows  north- 
ward to  the  land.    South-west  winds  will  therefore  arise. 
(Art.  99,)  which  prevail  from  the   peninsula  of  India  to 
the  Arabian  gulf,  until  the  time  of  the  autumnal  equinox. 
During  the  same  period,  the  south-east  monsoon,  in  the 
southern  hemisphere,  tempers  the  heat  of  Lower  Guinea, 
and  brings  rain  to  the  shares  of  Brazil. 

107.  The  motions  of  the  atmosphere,  however,  are 
reversed,  as  the  sun  crosses  the  equator  and  approaches 
the    southern   tropic.      Pouring  his    fervid   rays    upon 
Southern  Africa,  the  vast  tract  of  New  Holland,  and 
the  splendid  clime  of  Brazil,  the  air  flows  in  from  the 
north  and  north-west,  towards  these  highly  heated  re- 
,fions;  and  winds  from  these  quarters  prevail  for  several 


What  modifies  the  general  rule?    Give  examples. 
How  are  the  monsoons  caused  1 

3 


50  AERIAL    PHENOMENA 

months:  the  monsoon  extending  along  the  coast  of 
Brazil,  from  Cape  St.  Augustine  to  the  Isle  of  St.  Cath- 
erine. But  now  the  influence  of  the  sun  is  partially 
withdrawn  from  Southern  Asia  ;  it  glows  no  longer  be- 
neath his  vertical  rays,  and  over  the  cooled  earth  the 
north-east  monsoon  resumes  its  wonted  course. 

108.  LAND  AND  SEA  BREEZES.     On  the  coasts  and 
islands  within  the   tropics  a  sea  breeze  daily  occurs, 
about  nine  o'clock  in  the  morning ;.  at  first,  gently  blow- 
ing towards  the  shore,  but  gradually  increasing  in  force 
until  the  middle  of  the  day,  when  it  becomes  a  brisk 
gale  ;   after  two  or  three  o'clock  it  begins  to  subside, 
and  is  succeeded  at  evening  by  the  land  breeze,  which 
blows  freshly  off  the  coast  during  the  night,  dying  away 
in  the  morning,  when  the  sea  breeze  recommences. 

The  extent  of  these  winds  is  variable  ;  in  some  places 
they  are  scarcely  noticed  beyond  the  rocks  that  line  the 
beach  ;  at  others  they  are  perceptible  three  or  four 
leagues  from  land ;  while  such  is  their  strength  on  the 
Malabar  coast,  that  their  effects  are  felt  at  the  distance 
of  twenty  leagues  from  shore. 

These  breezes  are  occasionally  met  with  in  every 
latitude.  They  are  perceived  upon  the  coasts  of  the 
Mediterranean,  are  sometimes  felt  at  Bergen,  in  Nor- 
way, and  even  faintly  discerned  on  the  shores  of  Green- 
land. 

109.  ORIGIN.     During  the  day,  the  islands  of  the 
tropics  acquire  a  far  more  elevated  temperature  than  the 
adjacent  ocean ;  (Art.  91,)  the  atmosphere  above  them 
partakes  of  their  warmth,  and  currents  of  rarefied  air 
ascend  from  the  interior  of  the  land.    To  supply  the  par- 
tial void  thus  created,   the  cool,  dense  air  of  the  ocean 
llows  in  from  every  quarter  towards  the  shore,  and  the 
sea  breeze  then  prevails. 

About  mid-day  the  sea  breeze  is  strongest,  since  the 
velocity  and  force  of  the  ascending  current  is  then  at  its 
height,  for  the  sun  now  acts  with  its  greatest  energy  j 


Describe  the  land  and  sea  breezes.    Where  do  they  prevail  1 
How  do  they  originate  ? 


VARIABLE    WINDS.  51 

but  as  this  luminary  descends  in  the  heavens,  and  sinks 
beneath  the  waves,  the  land  rapidly  loses  its  heat  by 
radiation,  while  the  temperature  of  the  ocean  at  its  sur- 
face is  scarcely  changed.  This  is  owing  to  the  fact 
already  stated,  that  the  rays,  warm  only  the  surface  of 
the  earth,  but  are  diffused  through  the  water  to  a  con- 
siderable depth  ;  and  besides,  whenever  the  upper  stra- 
tum of  the  fluid  is  cooled,  it  becomes  heavier  and  sinks  ; 
and  a  warmer  stratum  rising  to  the  top  the  surface  thus 
maintains  an  almost  unvarying  temperature.  For  these 
reasons,  the  land,  at  length,  becomes  colder  than  the  sea, 
while  the  air  above  it,  acquiring  its  temperature,  is  con- 
densed, and  flowing  off  in  every  direction  to  the  warm 
and  lighter  atmosphere  that  floats  above  the  ocean, 
gives  rise  to  the  land  breeze,  which  prevails  throughout 
the  night. 

110.  VARIABLE  WINDS.    From  the  extreme  mobility 
of  the  air,  the  direction  of  the  wind  is  affected   bv  a 
countless  variety  of  causes,  such  as  the  nature  of  the 
soil,  the  inequalities  of  its  surface,  the  vicinity  of  the 
ocean  and  of  lakes  /  and  the  temperature,  course  and 
proximity  of  mountains. 

These  local  influences  are,  for  the  most  part,  con- 
trolled, where  the  great  aerial  currents  exist  in  all  their 
power  ;  but  in  the  extra-tropical  regions,  where  the  force 
of  the  latter  is  diminished,  a  perpetual  contest  occurs 
between  the  permanent  and  temporary  currents,  giving 
rise  to  constant  fluctuations  in  the  strength  and  direction 
of  the  wind. 

111.  It  appears  from  observations  made  at  Toronto, 
and  at  Hudson,  Ohio,  that  although   the  wind  blows 
from  every  point  of  the  compass  during  the  year,  yet, 
such  is  the  force  of  the  northerly  gales,  that,  in  these 
latitudes,  there  is  a  general  motion  of  the  atmosphere 
from  N.  W.  to  S.  E.    In  England,  on  the  contrary,  from 
the  hourly  observations  made  at  Plymouth,  there  seems 

In  what  regions  do  variable  winds  prevail  ?    What  appears  to  be  tha 
general  course  of  the  atmosphere  at  Toronto  and  Hudson  ? 
What  in  England  ? 


52  AERIAL     PHENOMENA. 

to  be  an  annual  movement  of  the  atmosphere,  from  the 
S.  S.  E.  towards  the  N.  N.  W. 

112.  PHYSICAL  NATURE  OF  WINDS.   Winds  are  hot, 
cold,  dry  or  moist,  according  to  the  direction  whence  they 
blow,  and  the  kind  of  surface  over  which  they  pass.    In 
Europe  the  westerly  winds  are  warm  and  moist,  and  the 
north-easterly  cold  and  dry  ;  for  the  former  come  over 
the  sea  from  the  lower  latitudes,  and  the  latter  sweep 
across  the  land  from  the  polar  climes :  in  our  own  cli- 
mate, a  north-easterly  wind  is  cold  and  moist. 

A  south  wind  in  the  northern  hemisphere  is  warm 
and  humid,  since  it  comes  from  warmer  regions,  and  its 
capacity  for  moisture  is  constantly  diminishing  in  its 
northward  progress ;  from  opposite  causes  a  north  wind 
is  keen  and  dry. 

In  the  southern  hemisphere  the  nature  of  these  winds 
would  be  interchanged. 

113.  PUNA  WINDS.    In  Peru,  between  the  Cordilleras 
and  the  Andes,  at  the  height  of  12,000  feet,  are  vast 
tracts  of  desolate  table-land,  known  by  the  name  of  the 
Puna.     These  regions  are  swept,  for  four  months  in 
the  year,  by  a  piercingly  cold  wind   from  the  snowy 
peaks  of  the  Cordilleras,   which  is  so  extremely  dry, 
and  absorbs  with  such  rapidity  the  moisture  of  animal 
bodies,  that  it  prevents  putridity.     If  a  mule  happens  to 
die  upon  these  plains,  it  is  converted,  in  the  course  of  a 
few  days:,  into  a  mummy,  even  the  entrails  being  free 
from  the  slightest  evidence  of  decay. 

According  to  Prescott,  the  ancient  Peruvians  pre- 
served the  bodies  of  their  dead  for  ages,  by  simply  ex 
posing  them  to  the  dry,  cold,  and  rarefied  atmosphere  of 
the  mountains. 

114.  SIMOOM.     Upon  the  arid  plains  of  Asia,  and  es 
pecially  on  the  vast  deserts  of  Africa,  an  intensely  hot 

Whence  arise  the  differences  In  the  properties  of  wind  ? 
What  is  the  nature  of  a  south  wind  in  the  northern  hemisphere,  and 
why  }    Of  a  north  wind,  and  why  ? 

Why  would  their  properties  be  reversed  in  the  southern  hemisphere! 
Describe  the  Puna  winds.    What  fact  is  stated  by  Prescott  1 


NATURE    OF    WINDS.  53 

wind  occasionally  prevails.  In  Arabia  and  Syria,  it  is 
known  by  the  name  of  the  simoom,  from  the  Arabic  word 
samma,  signifying  at  once  hot  and  poisonous.  In  Egypt 
it  is  termed  chamsin,  fifty,  because  it  usually  continues 
fifty  days ;  while  in  the  western  parts  of  the  great 
Zahara,  along  the  Senegal,  and  upon  the  coast  of 
Guinea,  it  receives  the  name  of  harmattan. 

The  stones  of  the  Arabs,  and  the  accounts  of  the 
earlier  travelers,  in  regard  to  this  wind,  are  clothed 
with  marvelous  fictions.  It  is  described  as  a  poison- 
ous, fiery  blast,  that  instantly  destroys  life ;  none  ever 
surviving  the  effects  of  its  deadly  influence,  if  once  in- 
haled. But  these  fables  are  now  exploded,  and  the 
simoom  is  known  to  possess  no  other  properties  than 
those  which  naturally  belong  to  an  exceedingly  hot  and 
parching  wind. 

115.  CAUSE.     Its  origin  is  to  be  sought  in  the  pecu- 
liarities of  the  soil,  and  the  geographical  position  of  the 
countries  over  which  it  reigns. 

The  surface  of  the  Asiatic  and  African  deserts  is 
composed  of  dry  quartz  sand,  which  the  powerful,  ver- 
tical rays  of  the  sun  render  burning  to  the  touch.  The 
heat  of  these  regions  is  insupportable,  and  their  atmos- 
phere like  the  breath  of  a  furnace.  In  June,  1813,  at 
Esne,  in  Upper  Egypt,  the  thermometer  of  Burckardt 
rose  to  120°  Fah.  beneath  the  roof  of  a  tent,  and 
in  1841,  the  British  embassy  to  the  king  of  Shoa,  while 
advancing  from  Tajura  to  Abyssinia,  suffered  under  a 
temperature  of  126°  Fah.  in  the  shade. 

When,  under  such  circumstances,  the  wind  rises  and 
sweeps  these  burning  wastes,  it  is  at  the  same  time  hot, 
and  extremely  destitute  of  moisture ;  and,  as  it  bears 
aloft  the  fine  particles  of  sand,  the  atmosphere  is  tinged 
with  a  reddish,  or  purple  haze,  the  sure  precursor  of 
the  simoom. 

116.  Though  the  blast  of  the  simoom  inflicts  not  in- 
stant death,  it  is  yet  a  dreadful  visitant  to  the  traveler 

What  is  the  simoom  ?    Where  does  it  prevail  1 

By  what  other  names  is  it  known  7    What  is  the  truth  respecting  it  1 

How  does  it  originate  1 


54  AERIAL     PHENOMENA 

of  the  desert.  Clouds  of  glowing  sand,  at  times  so 
thick  that  objects  are  invisible  at  the  distance  of  a  few 
paces,  are  driven  with  blinding  force  against  the  face  ; 
the  mojsture  is  rapidly  absorbed  from  the  body,  the  skin 
becomes  parched,  the  throat  inflamed,  respiration  is 
accelerated,  and  a  raging  thirst  created.  And  in  the 
midst  of  these  horrors,  the  burning  blast  deprives  its 
unhappy  victims  of  the  only  means  which  they  possess 
for  alleviating  their  sufferings  ;  the  water  evaporates 
through  the  skins  in  which  it  is  carried,  arid  whole 
caravans  have  been  known  to  perish,  the  prey  of  a  con- 
suming thirst. 

117.  SIROCCO.  This  name  is  given  to  a  south-east 
wind  which  prevails  in  the  Mediterranean  isles,  and 
along  the  Italian  shores.  During  the  summer  and 
autumn  it  is  peculiarly  distressing  to  the  inhabitants 
of  these  regions ;  an  oppressive  sensation  of  heat  is  then 
felt,  the  skin  is  bathed  in  perspiration,  the  body  becomes 
weak  and  languid,  and  the  mind  dispirited.  These 
effects  are  attributed  to  the  fact,  that  the  sirocco,  at  this 
time,  is  both  hot  and  moist;  very  little  evaporation 
therefore  occurs,  and  the  sensations  experienced,  under 
these  circumstances,  are  similar  to  those  which  are  felt 
during  a  very  sultry  state  of  the  atmosphere.  While 
this  wind  prevails,  the  air  is  obscured  by  fine  particles 
of  dust,  and  is  always  hazy. 

The  sirocco  has  been  generally  supposed  to  arise 
from  a  current  of  air  flowing  across  the  Mediterranean 
from  the  glowing  sands  of  Africa.  It  acquires  its  heat 
from  the  desert,  and  its  moisture  from  the  sea. 


CHAPTER  II. 

OP   HURRICANES. 

118.    HURRICANES  are  terrific   storms,  accompanied, 
at  times,  by   thunder  and  lightning ;    and  differ  from 

Describe  its  effects.     Describe  the  Sirocco. 

What  does  chapter  second  treat  of?    What  are  hurricanes  1 


HURRICANES. 


55 


Fig.  7. 


every  other  kind  of  tempest  by  their  extent,  their  irre- 
sistible power,  and  the  sudden  changes  that  occur  in  the 
direction  of  the  wind.  Though  known  in  other  climes, 
they  rage  with  the  greatest  fury  in  the  tropical  regions  ' 
The  rich  products  of  the  plantations  are  destroyed  in  a 
moment,  forests  are  leveled,  the  firmest  edifices  pros- 
trated and  their  roofs  whirled  aloft  into  the  air,  which 
is  filled  with  the  flying  fragments  of  a  thousand  ruins. 
Upon  the  coasts,  the  waves  rush  landward  with  appall- 
ing violence,  lining  the  harbors  and  the  adjacent  shores 
with  the  cargoes  and  wrecks  of  shattered  vessels. 

119.  From  the  late  independent  investigations  of  sev- 
eral eminent  philosophers,  it  also  appears  that  hurri- 
canes are  extensive  storms  of 

wind,  which  revolve  around  an 
axis,  either  upright  or  inclined 
to  the  horizon ;  while  at  the 
same  time,  the  body  of  the  storm 
has  a  progressive  motion  over 
the  surface  of  the  globe. 

120.  We  learn  from  the  nu- 
merous   observations    collected 
by  Mr.  Redfield,  of  New  York, 
that    in    the    northern    hemi- 
sphere, the  Atlantic  hurricanes 
generally  originate  to  the  east 
of  the   Carribean  islands,  and 
that  their  path  is  from  south- 
east  to  north-west,   until  they 
have  passed  the  northern  tropic, 
when  their  course  changes  from 
south-west   to   north-east;   the 
rotation  of  the  storm  being  from 
right  to    left,  contrary   to   the 
motion  of  the  sun,  (see  fig.  7.,  OENE 
where  the  arrows  show  the  di- 
rection of  the  wind.) 

Where  are  they  most  violent  1  What  do  many  philosophers  now  con- 
sider them  to  be  ? 

State  Mr.  Redfield's  views  in  regard  to  the  Atlantic  hurricanes  of  the 
•northern  hemisphere. 


NORTHERN   HEMISPHERE. 


56 


AERIAL    PHENOMENA. 


Fig  8. 


The  researches  of  Col.  Reid, 
the  Governor  of  the  Bermudas, 
have  likewise  shown,  that  the 
storms  and  tempests  of  the 
southern  latitudes  are  vast 
?/•// irlirin ds  ;  moving,  however, 
in  ;i  different  manner  from  the 
hurricanes  of  the  northern  hem- 
isphere. Thus,  south  of  the 
equator,  the  general  course  of 
the  hurricanes  is  from  the  north- 
east to  the  south-west j  within 
the  southern  tropic;  but  after 
passing  this  limit  they  proceed 
from  the  north-west  to  the  south- 
east •  revolving  from  left  to 
right,  in  the  same  way  as  the 
sun;  a  fact  previously  conjec- 
tured by  Mr.  Redfield.  (See 
fig.  8.) 

The  hurricanes  of  the  southern 

hemisphere    frequently    OCCUr    in    OEXERAI,  DIRECTION   AND  ROTA- 

.  \      .      .  r       AT  J          T10N    OF    HURRICANES    IN    THB 

the      Vicinity     of     Mauritius     and  SOUTHERN  HEMISPHERE 

Madagascar. 

121.  PATH  OF  THE  STORM.  The  distance  traversed 
by  these  desolating  tempests  is  immense.  The  memo 
rable  gale  of  August,  1830,  which  fell  upon  St.  Thomas, 
in  the  West  Indies,  on  the  12th,  reached  the  Banks  of 
Newfoundland  on  the  19th  ;  having  traveled  more  than 
t/irrr  tltoiiwinil  nautical  miles  in  seven  days ;  and  the 
observed  track  of  the  Cuba  hurricane  of  1844  was  but 
little  inferior  in  length. 

122.  VELOCITY.  Their  progressive  velocity  varies  on 
the  Atlantic  Ocean,  from  seventeen  to  thirty  miles  per  hour ; 
but  at  certain  portions  of  the  track  it  is  sometimes  much 
higher;  as  in  the  case  of  the  Cuba  hurricane,  where  the 


State  Col.  Reid's  views  in  respect  to  those  of  the  southern. 
What  is  said  of  the  distance  traversed  by  hurricanes? 
What  of  their  progressive  and  rotary  velocity! 


HURRICANES.  57 

average  rate  from  the  Bahamas  to  45°  N.  Lat.  was  forty 
miles  per  hour.  Distinct  from  the  progressive  is  the  ro- 
tary velocity,  which  increases  from  the  exterior  boundary 
to  the  centre  of  the  storm,  near  which  point  the  tempest 
rages  with  terrific  force  ;  the  wind  sometimes  blowing  at 
the  rate  of  one  hundred  miles  per  hour. 

123.  DIAMETER.     The  surface  simultaneously  swept 
by  these  tremendous  whirlwinds  is  a  vast  circle,  varying 
from  one  hundred  to  five  hundred  miles  in  diameter ; 
but  even  the  greatest  of  these  dimensions  was  exceeded 
in  the  Cuba  hurricane,  for  its  breadth  was  computed  by 
Mr.  Redfield    to  be  at  least  800  miles,  and   the  area 
over  which  it  prevailed,  throughout  its  whole  length, 
2,400,000  square  miles  ;  an  extent  of  surface  equal  to 
two-thirds  of  that  of  all  Europe. 

124.  The  rotary  character  of  the  hurricane  accounts 
for  the  frequent  changes  that  occur  in  the  direction  of 
the  wind ;  since,  in  order  to  preserve  a  circular  motion, 
there  must  be   a  constant   deflection    from  a  straight 
course,  and,  at  corresponding  points  in  each  half  of  the 
storm,  the  gale  will  blow  from  opposite  quarters.     The 
changes  thus  caused,  will  be  perceived  at  any  spot  over 
which  this  fearful  visitant  passes. 

It  also  explains  the  fact,  that  the  violence  of  the  wind 
increases  towards  the  centre,  and  that,  within  the  very 
vortex  of  the  hurricane,  the  air  is  in  repose.  Here  oc- 
curs that  awful  calm,  described  by  mariners  as  the  lull 
of  the  tempest,  in  which  it  seems  to  sleep,  only  to  gather 
strength  for  mightier  conflicts. 

125.  CASES.     Numerous  instances  of  the  facts  above 
mentioned  might  be  adduced,  but  one  or  two  will  suffice. 
In  the  Antigua  hurricane  of  1837,  described  by  Col. 
Reid,  it  appears  that  Capt.  Newby  of  the  Water  Witch, 
first  experienced  its  effects  at  St.  Thomas,  in  the  West 
Indies,  on  the  morning  of  the  second  of  August.     The 
wind  was  then  N.  N.  W.,  and  at  three  in  the  afternoon 

How  great  is  their  breadth  1 
How  great  the  surface  over  which  they  prevail  1 
What  facts  are  explained  by  \he  rotation  of  the  storm  1 
Give  instances. 

3* 


68  AERIAL    PHENOMENA. 

became  violent.  At  five  P.  M.  it  blew  a  severe  gale, 
and  at  seven  P.  M.,  says  Capt.  Newby,  "a  hurricane 
arose  beyond  description  dreadful.  Soon  after  a  calm 
succeeded  for  about  ten  minutes,  and  then,  in  the  most 
tremendous  screech  I  ever  heard,  it  recommenced  from 
the  S.  and  S.  W.  At  two  o'clock  on  the  morning  of  the 
third,  the  gale  somewhat  abated,  and  the  barometer  rose 
an  inch.  At  daylight,  out  of  forty  vessels,  the  Water 
Witch  and  one  other  were  the  only  two  not  sunk, 
ashore,  or  capsized." 

126.  On  the  12th  of  August,  1837,  another  hurricane 
commenced,  in  the  same  region,  in  17°  N.  Lat.  and  53° 
45'  W.  Lon.   At  midnight  on  the  18th,  in  31°  N.  Lat.,  the 
ship   Rawlin,    Capt.  Macqueen,  appears,  according   to 
Col.  Reid,  to  have  been  in  the  very  vortex  of  the  storm. 
On  the  17th,  the  wind  blew  strong  from  the  N.  E.  by 
E.  for  twelve  hours,  then  suddenly  changed  to  the  north, 
blowing  with   undiminished  violence   till  the  18th  at 
midnight  when,  in  an  instant,  a  perfect  calm  ensued  for 
the  space  of  one  hour  ;  then,  "  quick  as  thought,  the  hur- 
ricane sprung  up  with  tremendous  force  from  the  S.  W. ; 
no  premonitory  swell  of  the  wind  preceding  the  convul- 
sion."    During  the  gale,  the  barometer  was  almost  in- 
visible in  the  tube  above  the  framework  of  the  instru- 
ment. 

The  sudden  and  extraordinary  transition  detailed  in 
the  cases  just  cited,  are  fully  explained  by  supposing, 
that  the  vessels  passed  from  one  side  of  the  whirl  to  the 
other,  through  the  vortex  of  the  tempest. 

127.  FALL  OF  THE  BAROMETER.     If  the  hurricane 
is  indeed  a  vast  whirlwind,  the  atmosphere,  constituting 
the  body  of  the  storm,  will  be  driven  outward  from,  the 
centre  towards  the  margin  (C.  171),  just  as  water  in  a 
pail,  which  is  made  to  revolve  rapidly,  flies  from  the 
centre,  and  swells  up  the  sides.     But  the  pressure  of 
the  atmosphere,  beyond  the  whirl,  checking,  and  resist- 
ing this  centrifugal  force,  at  length  arrests  the  outward 
progress  of  the  aerial  particles,  and  limits  the  storm. 

If  the  hurricane  is  a  whirlwind,  in  what  manner  should  the  barometer 
fall  and  rise? 


HURRICANES.  59 

We  should  consequently  expect  to  find  (in  accordance 
with  the  laws  of  circular  motion)  the  density  of  the  air 
increasing-  from  the  centre  to  the  circumference  of  the 
storm,  and  even  for  some  distance  beyond  its  boundary ; 
and  likewise,  that  when  a  hurricane  passed  diametric- 
ally over  any  region,  the  atmospheric  pressure  would 
decrease,  and  the  barometer  continue  to  sink,  during  the 
first  half  of  the  storm  ;  but  that  the  instrument  would 
gradually  rise,  as  the  last  half  passed  over.  Such 
indeed  is  the  case  ;  for,  amid  all  the  phenomena  of 
storms,  no  fact  is  better  established  than  this,  that  an 
extraordinary  depression  of  the  barometer  in  tropical 
climates  is  a  sure  forerunner  of  a  hurricane. 

128.  Before  the  tempest  of  Aug.  2d,  1837,  the  harbor- 
master of  Porto  Rico  warned  the  shipping  in  port  to 
prepare  against  a  storm,  as  the  barometer  was  falling  in 
an  unusual  manner  ;  having  sunk  one  and  a  half  inches 
since  8  o'clock  in  the  evening  of  the  preceding  day. 
All  precautions  were  however  in  vain  ;  thirty-three  ves- 
sels at  anchor  were  destroyed,  and,  at  St.  Bartholo- 
mews, twro  hundred  and  fifty  buildings  levelled  to  the 
earth.  The  following  table  of  observations,  taken  at  St. 
Thomas,  over  which  island  this  hurricane  passed,  is  full 
of  instruction  in  regard  to  this  important  point. 

IB  tliii  the  case  J    Relate  the  instances  given. 


60 


AERIAL     PHENOMENA 


129. 


TIME. 

HEIGHT   OF 
THE    BAROMETER. 

WIND. 

Hours  and  Minutes. 

Inches. 

Direction  and  Force. 

Aug.  2d,  A.M.    6 
P.M.    2    10 
3    45 
4    45 

29.95 
29.77 
29.69 
29.51 

r 

Increasing 
Tempest. 

5    45 

29.33 

\.  E. 

6    30 

29.16 

N.  VV. 

6    35 
6    45 

28.93 
28.80 

N.  W. 
\.  W. 

•Hurricane. 

7    10 

28.62 

\.  W. 

7    30 

28.18 

N.  W. 

7    35 

28.13 

7    52 

8    10 

28.09 
28.09 

•Dead  Calm. 

8    20 

28.09 

8    23 

28.44 

S.  S.  E. 

8    33 

28.53 

S.  E. 

8    38 

28.62 

S.  E. 

8    45 

28.71 

S.  E. 

8    50 

28.SO 

S.  E. 

9 

28.98 

S.  E. 

9    10 

29.16 

S.  E. 

•Hurricane. 

9    25 

29.24 

S.  E. 

9    35 

29.33 

S.  E. 

9    50 

29.42 

S.  E. 

10    10 

29.51 

S.  E. 

10    35 

29.60 

S.  E. 

11    30 

29.64 

S.  E. 

Aug.  3d.  A.  M.   2    45 

29.73 

S.  E. 

8 

29.91 

S.  W. 

9 

29.93 

E. 

130.  In  the  case  of  the  Water  Witch,  we  have  seen, 
that,  \vhen  the  centre  of  the  tempest  was  past,  and  the 
gale  abated,  the  barometer  rose  an  inch. 

131.  CIRCUIT  SAILING.    The  gyratory  motion  of  Imr 
ricanes  is  strikingly  evinced  by  vessels  -sailing  on  a  cir- 
cular course,   when   scudding  before    the   wind.      The 
most  remarkable  case  is   that  of  the   Charles  Ileddle, 
ivhti-d  by  -Mr.  Piddington,  which  occurred  in  a  storm, 
year  .Mauritius,  in  Feb.  1845. 

It  appears  from  the  log-book  of  this  ship,  that,  in  her 
course    before  the  gale  the  wind   changed    completely 


What  example  is  given  of  circuit  sailing  1 


HURRICANES.  61 

round  five  times  in  the  space  of  one  hundred  and  seven- 
teen hours,  having  an  average  velocity  of  eleven  miles 
and  seven-tenths  per  hour.  The  whole  distance  thus 
sailed  by  the  vessel  was  thirteen  hundred  and  seventy- 
three  miles;  while  her  actual  progress  during  this  time 
in  a  south-westerly  direction,  was  found  to  be  only  three 
hundred  and  fifty -four  miles. 

132.  Axis  OF  THE  HURRICANE.      The  axis  of  the 
hurricane  is  not,  necessarily,  upright,  but  is  usually  in- 
clined to  the  horizon  ;  leaning  in   the  direction  which 
the  tempest  takes.     This  is  owing  to  the  friction  of  the 
base  of  the  hurricane  against  the  surface  of  the  earth. 
Its  velocity  is  thus  checked,  while  the  upper  portion  is 
driven  forward,  and  overhangs  the  base. 

This  position  of  the  axis  is  indicated  by  the  circum- 
stance that  the  tokens  of  the  approaching  tempest  often 
appear  in  the  higher  regions  of  the  atmosphere,  before 
it  is  felt  below.  The  navigators  of  the  tropic  seas  some- 
times behold,  high  in  the  air,  a  small  black  cloud  ; 
rapidly  it  spreads  down  to  the  horizon,  shrouding  sea 
and  sky,  and  the  tempest  then  suddenly  descends  upon 
them  in  all  its  fury. 

133.  REMARKS.     Such  are  the  opinions  entertained 
by  Redfield,  Reid,  Dove,  and  others,  in  regard  to  storms 
and  hurricanes;  opinions  based  upon  a  vast  assemblage 
of  facts   and   observations,  gathered   from    all   points, 
within  the  track  of  a  great  number  of  these  desolating 
gales.      The  numerous  observations  taken    upon    the 
American  coast,  commensurate  with  the  extent  of  the 
Atlantic  tempests,  have  been  systematized  by  Mr.  W. 
C.  Redfield,  of  New  York ;  while  Col.  Reid  has  investi- 
gated the    West  India  hurricanes,   and   those    of   the 
southern   hemisphere,  with    great   success.      The  log- 
books of  the  British  navy,  in  which  the  phenomena  of 
the  weather  are  recorded  every  half  hour,   have  been 


What  is  the  position  of  the  axis  of  the  hurricane  1 
How  is  it  caused  ? 

How  is  this  position  sometimes  :ndicated  ? 
Detail  the  labors  of  Rediield,  Reid,  and  Dove. 


62  AERIAL     PHENOMENA. 

placed  at  his  disposal,  and  he  has  thus  been  furnished 
with  an  immense  collection  of  valuable  facts.  Prof. 
Dove,  of  Berlin,  has  studied  the  laws  of  hurricanes  in 
Europe,  and  gathered  a  large  number  of  observations 
from  every  quarter  of  the  globe.  By  noticing  the  time 
and  place  of  each  observation,  storm-charts  have  been 
constructed  for  the  use  of  mariners,  and  it  is  highly  in 
favor  of  the  rotary  theory,  that  the  conclusions  result 
ing  from  these  extensive  and  independent  investigations 
are  substantially  the  same. 

134.  ESPY'S  THEORY.  The  rotary  character  of  hur- 
ricanes, including  tornadoes  and  water-spouts,  is  how- 
ever denied  by  Mr.  Espy,  of  Philadelphia,  who  main- 
tains that  the  wind  blows  from  every  quarter  towards 
the  centre  of  the  storm.  Espy  asserts,  that  this  law  ob- 
tains without  a  single  exception,  in  seventeen  storms 
wbich  he  has  investigated.  The  influx  of  wind  towards 
the  centre,  he  supposes  to  be  caused  by  the  development 
of  heat,  which  occurs  whenever  atmospheric  vapor  is 
condensed  in  the  form  of  a  cloud.  The  heat,  thus  dia- 
engaged,  rarefies  the  surrounding  air,  and  establishes 
an  upward  current ;  and  so  great  an  expansion  is  be- 
lieved, at  times,  to  result  from  this  cause,  that  the  ve- 
locity of  the  ascending  current  has  been  computed  to 
exceed  three  hundred  and  fifty  feet  per  second. 

To  this  point  of  greatest  rarefaction,  the  atmosphere 
rushes  in  from  every  side,  just  as  the  air  of  a  room 
flows  towards  the  heated  current  of  the  chimney ;  the 
violence  of  the  wind  depending  upon  the  rate  of  speed 
in  the  ascending  column.  Most  of  the  phenomena  of 
meteorology  are  also  explained  by  Mr.  Espy  in  accord- 
ance with  his  peculiar  views. 

135.  The  centripetal    theory  has  found    many  able 
supporters  ;    but  that   of   Redfield  and  Reid  has  been 
more  generally  adopted  by  men  of  science. 

136.  It  may  perhaps  be  found,  \vhen  our  investiga- 
tions are  multiplied  and  more  extended,  that  both  these 

Detail  Mr.  Espy's  theory. 

Which  theory  has  been  more  generally  adopted  7 

May  these  two  motions  co- exist  1 


TORNADOES    OR    WHIRLWINDS.  63 

motions  often  co-exist ;  a  circumstance  which  is  by  no 
means  impossible.  For  when  a  whirlwind  is  once  in 
motion,  from  any  cause  whatsoever,  the  great  rarefac- 
tion of  air  that  occurs  at  the  centre,  will  create  an 
influx  of  the  atmosphere  towards  this  point  from  all 
quarters,  except  where  it  is  opposed  by  the  centrifugal 
force.  Now  if  the  base  of  the  whirl  is  above  the  sur- 
face of  the  earth,  or  when  touching  it,  is  inclined  to  it, 
(which  is  usually  the  case,)  currents  of  air  will  flow 
beneath  the  base  towards  the  vortex,  and  evidences  of 
centripetal  action  will  not  be  wanting. 


CHAPTER   III. 

OF   TORNADOES   OR  WHIRLWINDS. 

137.  Tornadoes  may  be  regarded  as  hurricanes,  dif- 
fering chiefly  in  respect  to  their  extent  and  continuance. 
They  last  only  from  fifteen  to  sixty  or  seventy  seconds, 
their  breadth  varies  from  a  few  rods  to  several  hundred 
yards,  and  it  is  probable  that  the  length  of  their  track 
rarely  exceeds  twenty-five  miles. 

138.  FACTS.     This  phenomenon  is  usually  preceded 
by  a  calm  and  sultry  state  of  the  atmosphere ;  when  sud- 
denly the  whirlwind   appears,  traversing  the  earth  with 
great  velocity,  and  sweeping  down  by  its  tremendous 
power  the  mightiest  products  of  nature,  and  the  strongest 
works  of  man.     Ponderous  bodies  are  whirled  aloft  into 
the  air  ;  trees  of  large  dimensions  twisted  off  or  torn  up 
by  the  roots  ;  buildings  of  the  firmest  construction  pros- 
trated, and  streams  whirled  from  their  beds  and  their 
channels   laid   bare.      A  whirlwind    that   occurred   in 
Silesia,  in  the  year  1820,  carried  a  mass  weighing  more 
than  650  lbs.:Jifti/  feet  above  the  top  of  a  house,  and 

What  are  tornadoes  1 
Describe  their  effects. 
By  what  phenomena  are  they  attended  1 


C4  AERIAL    PHENOMENA. 

deposited  it  on  the  other  side  in  a  ditch,  one  hundred 
and  fifty  paces  distant. 

139.  in  1755  a  tornado  fell  upon  the  village  of  Mira- 
oeau,  in  Burgundy,  laying  dry  the  channel  of  the  small 
river  by  which  it  is  traversed,  and  carrying  the  stream 
to  the  distance  of   sixty  paces.      In  the  New  Haven 
whirlwind  of  1839,  and  in  that  which  occurred  at  Chate- 
nay,  near  Paris,  during  the  same  year,  trees  eighteen 

.inches  in  diameter  were  torn  up  by  the  roots.  In  one 
which  happened  at  Maysville,  Ohio,  in  1842,  a  barn 
containing  three  tons  of  hay  and  four  horses,  was  lifted 
entirely  from  its  foundations.  And  such  was  the  force 
of  the  wind  during  a  tornado  which  occurred  at  Cal- 
cutta in  1833.  that  a  bamboo  was  driven  quite  through 
a  wall  five  feet  thick,  covered  with  masonry  on  both 
sides  ;  an  effect  which  was  estimated,  by  a  person  on 
the  spot,  to  be  equal  to  that  produced  by  a  cannon  car- 
rying a  six-pound  ball. 

By  the  action  of  a  tornado,  fowls  are  often  entirely 
stripped  of  their  feathers,  and  light  substances  carriec1 
to  a  distance  varying  from  two  to  twenty  miles. 

140.  The  whirlwind    is  attended   by  all  the    usual 
phenomena  of  thunder-storms ;    showers   of    hail    fre- 
quently occur,  and,  at  times,  it   is  the  scene  of  very 
extraordinary  electric  appearances.     In  the  one  which 
happened  at  Morgan,  Ohio,  on  the  night  of  the  19th  of 
June.  1823,   a  bright  cloud  of  the  color  of  a  glowing1 
oven,  and  apparently  half  an  acre  in  extent,  was  seen 
moving  below  the  dark  canopy  of  the  tempest.    It  shone 
with  a  splendor  above  that  of  the  full  moon,  and  ten 
minutes  after  its  passage,  the  narrator  of  the  phenomena 
was  enabled  to  read  his   Bible  by  its  light.     Just  before 
the  Shelbyville  tornado,  which  took  place  at  midnight, 
on  the  31st  of  May,  1830,  two  luminous  clouds  were 
seen  approaching  each  other,  of  the  color  of  red  hot 
iron  ;  for  a  moment  they  united  above   the  town,  ex- 
tending over  it   like  two  fiery  wings,  and,  at  the  next, 
rushed  down  to  the  earth  :  at  this  instant  the  whirlwind 
burst  in  all  its  fury  upon  the  devoted  spot.     The  writer 

What  extraordinary  appearances  are  sometimes  seen  1 


TORNADOES    OR    WHIRLWINDS.  65 

of  this  work  was  informed  by  an  eye-witness,  that,  dur- 
'  ing  the  prevalence  of  the  storm,  so  incessant  was  the 
play  of  the  lightning,  that  the  titles  of  books  could  be 
easily  read,  and  the  use  of  lamps  was  discarded  in  going 
to  different  parts  of  the  house. 

141.  ORIGIN.     Several  theories  have  been  advanced 
to  explain  the  causes  of  whirlwinds,  but  they  are  sup- 
posed to  be  generally  produced  by  the  lateral  action  of 
opposing"  winds,  or  the  influence  of  a  brisk  gale  upon  aO 
portion  of  the  atmosphere  in  repose  ;  in  a  manner  anal- 
ogous to  the  eddies  that  arise  at  the  junction  of  two 
streams,   flowing  with   unequal  velocities,  or    the   air- 
whirls  that  occur,  when  a  wind  sweeps  by  the  corner  of 
a  building,  and  strikes  the  calm  air  beyond  it. 

142.  The  existence  of  such  opposing  currents  is  fully 
proved  by  the  observations  of  aeronauts,  as  well  as  by 
those  of  observers  at  the  surface  of  the  globe. 

The  whirl  appears  to  originate  in  the  higher  regions 
of  the  atmosphere,  and  as  it  increases  in  violence,  to 
descend  ;  its  base  gradually  approaching  until  it  touches 
the  earth. 

Thus,  when  on  the  summit  of  the  Rigi — a  mountain 
in  Switzerland — Kaemtz  beheld  two  masses  of  fog  ap- 
proaching each  other,  in  the  valley  of  Goldan,  while  the 
air  around  him  was  calm,  and  the  sky  serene.  As  soon 
as  they  united,  a  gyratory  motion  was  perceived,  the 
fog  rapidly  extended,  accompanied  with  violent  gusts  of 
rain  and  hail.  At  the  same  time,  (as  appeared  from 
subsequent  information,)  a  furious  storm  fell  upon  the 
lake  of  the  Four  Cantons,  far  below  ;  in  the  midst  of 
which  a  water-spout  was  seen.  (Art.  150.) 

Sf  143.  WHIRLWINDS  EXCITED  BY  FIRES.     Extensive 
conflagrations  have  been   known  also  to  produce  whirl- 
winds,  in   consequence  of  the  strong  upward   current, 
resulting  from  the  great  expansion  of  the  heated  air. 
A  remarkable  instance  of  this  kind  occurred  between 


How  do  they  originate,  and  where  1 

What  did  Kaemtz  witness  ? 

What  is  the  effect  of  extensive  fires? 


06  AERIAL     PHENOMENA. 

Great  Barrington  and  Stockbridge,  Mass.,  in  the  month 
of  April,  1783,  and  is  thus  related  by  Mr.  T.  Dwight, 
who  beheld  it.  "  In  an  open  field,  a  large  quantity  of 
nrush-wood  was  lying  in  rows  and  heaps  for  burning, 
perfectly  dry  and  combustible.  On  a  certain  day.  when 
the  atmosphere  was  entirely  calm,  the  brush  was  ignit- 
ed on  all  sides  of  tbe  field  at  once.  I  was  residing  at 
this  time,  at  the  distance  of  about  half  a  mile  from  the 
fire,  when  suddenly  my  attention  was  aroused  by  a  loud, 
roaring  noise,  like  heavy  thunder.  Upon  going  to  the 
door,  I  beheld  the  fire  covering  the  field,  and  the  flames 
collected  from  every  side  into  a  fiery  column,  broad  at 
tbe  base,  tapering  upward,  and  extending  to  the  height 
of  15U  or  200  feet.  This  pillar  of  flame  revolved  with 
an  amazing  velocity,  while  from  its  top  proceeded  a 
spire  of  black  smoke,  to  a  height  beyond  the  reach  of 
tbe  eye,  and  whirling  with  the  same  velocity  as  the  fiery 
column.  During  the  whole  period  of  its  continuance, 
the  column  of  flame  moved  slowly  and  majestically 
around  the  field.  The  noise  of  the  whirlwind  was 
louder  than  thunder,  and  its  force  so  great,  that  trees 
six  or  eight  inches  in  diameter,  which  had  been  cut, 
and  were  lying  on  the  ground,  were  whirled  aloft  to  the 
height  of  forty  or  fifty  feet." 

144.  During  the  terrible  conflagration  of  Moscow,  in 
1812,  the  air  became  so  rarefied  by  the  intense  heat,  that 
the  wind  rose  to  a  frightful  hurricane;  the  roar  of  the 
tempest  being  heard  even  above  the  rushing  sound  of 
the  conflagration. 

115.  RESULTS  OP  CENTRIFUGAL  ACTION.  By  the 
centrifugal  action  of  the  whirl,  tbe  air  is  driven  outward, 
as  in  the  case  of  hurricanes,  and  at  the  same  time  spi- 
rally upwards^  on  account  of  the  pressure  of  the  sur- 
rounding atmosphere:  a  great  rarefaction,  therefore, 
occurs  at  the  centre.  As  long  as  the  base  of  the  whirl- 
wind is  above  the  ground,  the  warm  air  of  the  earth 
will  stream  under  and  upwards,  into  this  partial  void 


Give  the  cases. 

Slate  the  result  of  centrifugal  action. 


TORNADOES    OR    WHIRLWINDS.  67 

from  every  quarter ;  while,  at  the  same  time,  the  cold 
air  will  descend  into  it  from  the  higher  region  of  the 
atmosphere. 

By  this  union,  a  powerful  condensation  of  vapor  oc- 
curs ;  causing  the  precipitation  of  rain  and  hail,  and 
the  development  of  electricity. 

146.  These,  however,  constitute  no  essential  part  of  a 
whirlwind  ;  for,  if  the  currents  of  air  that  give  rise  to 
this  pheuomenon  are  very  dry,  the  violence  of  the  wind 
is  the  only  remarkahle  circumstance.     This  was  shown 
in  the  case  of  a  small  whirl,  which  involved  two  persons, 
who  were  going  one    cloudy  day  from  Halle  to  Gie 
bichenstein.     Suddenly  they  were  separated  hy  a  gust 
of  wind  ;  one  being  driven  against  a  wall,  and  the  other 
thrown  into  a  field  ;  while  the  people  who  were  near 
had  not  discerned  the  slightest  disturbance  in  the  at- 
mosphere. 

147.  When  the  base  of  the  whirlwind  descends  to  the 
earth,  it  touches  the  surface,  either  partially  or  wholly, 
according  as  the  axis  is  inclined  or  vertical.    In  the  first 
case,  the  inward  flowing  currents  will  be  partially,  and 
in  the  second  entirely,  arrested  by  the  centrifugal  action 
of  the  storm. 

The  same  results  often  occur  when  it  covers  a  build- 
ing. Hence,  the  atmosphere  becomes  exceedingly  rare- 
fied, both  above,  and  around  the  edifice  ;  and  if  it  hap- 
pens to  be  closed,  and  the  tornado  is  violent,  its  walls 
will  be  burst  outward  by  the  sudden  expansion  of  the 
air  within,  (C.  509.)  Just  as  a  sealed  bottle  of  thin  glass, 
under  the  exhausted  receiver  of  an  air-pump,  is  shivered 
by  the  elastic  force  of  the  enclosed  air. 

148.  EFFECTS  OF  EXPANSION.     In  the  tornado  that 
happened   at   Natchez,  in   1840,  the   houses   exploded 
wherever  the  doors  and  windows  were  shut;  the  roofs 
shooting  up  into  the  air,  and   the  walls,  even  of  the 
strongest  brick  buildings,  bursting  outward  with  great 

Are  rain,  hail,  and  electricity  necessary  to  the  production  of  awhirlwind'l 

Give  the  case. 

Why  are  buildings  burst  outward  by  the  action  of  tornadoes  1 

Give  instances. 


68  AERIAL     PHENOMENA. 

force :  but  no  such  destruction  occurred  when  a  free 
outlet  was  afforded  to  the  air  within.  One  gentleman 
as  the  storm  approached,  caused  all  the  windows  and 
doors  of  his  house  to  be  opened,  and  though  its  struc- 
ture was  frail  it  experienced  no  injury ;  not  even  a 
single  pane  of  glass  being  broken. 

149.  On  the  18th  of  June,  1839,  a  whirlwind  (to 
which  we  have  alluded)  fell  upon  the  village  of  Chate- 
nay,  near  Paris.  In  the  room  of  a  house,  over  which  it 
passed,  several  articles  of  needlework  were  lying  upon 
a  table:  the  next  day  some  of  them  were  found  in  a 
field,  at  a  great  distance  from  the  house,  together  with  a 
pillow-case  taken  from  another  room.  They  must  have 
been  carried  up  the  chimney  by  the  rush  of  air  out- 
wards, as  every  other  means  of  exit  was  closed.  An- 
other singular  illustration  of  the  fact  before  us  took 
place  in  the  Shelbyville  tornado.  Soon  after  its  occur- 
rence, a  lady  missed  a  bonnet,  which,  the  day  before 
the  storm,  was  lying  enclosed  in  a  bandbox  in  her  cham- 
ber ;  some  weeks  afterwards,  she  accidentally  observed 
a  ribbon  hanging  from  the  chimney,  which  proved  to  be 
the  string  of  her  bonnet.  The  house  had  been  closed 
during  the  storm,  and  the  expansion  of  the  air  within 
the  bandbox  had  forced  off'  the  lid — the  lost  article  had 
been  borne  by  the  outward  flowing  current  up  the  chim- 
ney, which  afforded  the  only  mode  of  egress,  and  there 
it  had  lodged. 


CHAPTER   IV. 

WATER- SP  OUTS. 

J5U.  A  WATER-SPOUT  is  a  whirlwind  over  an  ex- 
IKIHSH  of  water,  as  the  sea  or  a  lake,  differing  from  a 
land-whirl  in.  no  other  respect  than  that  water  is  sub- 
jected to  its  action,  instead  of  the  bodies  upon  the  sur- 
face of  the  earth. 

151.   A   water-spout   usually  presents  the   following 

Define  a  water-spout. 


WATER-SPOUTS. 


successive  appearances.  At  first  it  is  seen  as  an  invert- 
ed cone,  either  straight  or  slightly  curved,  extending 
downward  from  a  dark  cloud  to  which  it  seems  to  be 
aitached.  As  the  cone  approaches  the  surface  of  the 
water,  the  latter  becomes  violently  agitated,  and,  rising 
in  spray  or  mist,  is  whirled  round  with  a  rapid  motion. 
As  the  cone  descends  lower  the  spray  rises  higher  and 
higher,  until  both  unite,  and  a  continuous  column  is 
formed  extending  from  the  water  to  the  clouds. 

The  spout  is  now  complete,  and  appears  as  an  im- 
mense tube,  possessing  both  a  rotary  and  progressive 
motion  ;  bending  and  swaying  under  the  action  of  the 
wind  as  it  advances  on  its  course. 

When  the  observer  is  near,  a  loud,  hissing  noise  is 
heard,  and  the  interior  of  the  spout  seems  to  be  traversed 
by  a  rushing  stream. 

After  continuing  a  short  time  the  column  is  disunited, 
and  the  dark  cloud  gradually  drawn  up  ;  for  a  while  a 
thin,  transparent  tube  remains  below,  but  this  at  last  is 
also  broken,  and  the  whole  phenomenon  then  disap 
pears.  These  successive  change*  are  represented  in 
figures  9, 10, 11,  which  are  taken  from  sketches  of  water- 
spouts actually  seen. 

Fig.  9. 


WATER-SPOUT   FORMING. 


What  are  its  successive  appearances  1 


70 


ITER-SPOUT   FORMED. 
Fig.  11. 


WATER-SPOUT   ENDING. 


152.  FACTS.     A  water-spout  occurred  at  Cleveland, 
Ohio,  in  September,  1835,  which,  from  the  description 


Describe  the  one  which  was  seen  at  Cleveland. 


WATER-SPOUTS.  71 

well  illustrates  the  origin  and  characteristics  of  this  phe- 
nomenon. "  A  heavy  storm-cloud,  driven  by  a  north- 
west gale,  was  met  by  a  strong  opposing  current; 
when  an  arm  of  the  cloud  appeared  to  drop  down,  and 
drag  the  waves  up  towards  the  sky.  The  whirling  and 
dashing  of  the  spray  at  the  surface  of  the  lake,  and  the 
column  of  water  and  mist  extending,  in  a  tall  and  tor- 
tuous line,  to  the  cloud,  were  so  well  denned  as  to  ex- 
cite the  admiration  of  all  who  observed  them.  At  the 
expiration  of  about  seven  minutes,  the  north-wester  tri- 
umphed, and  swept  the  cloud  to  the  south-east  of  the 
city." 

153.  The  water-spout  does  not  always  pass  through 
the  various  changes  that  have  been  detailed  ;  sometimes 
the   upper  portion  only  is  developed,  depending  from  a 
ma&s  of  black  clouds,  like  a  huge,,  tapering  trunk,  with- 
out ever  reaching  the  water  ;  at  other  times,  nothing  is 
seen  hut  the  cloud  of  spray  and  mist  that  forms  the  base. 
On  the  voyage  of  the  Exploring  squadron  from  New 
Zealand  to  Tongataboo,  a  spout  was  beheld  in  the  act 
of  foiming,  at  the  distance  of  about  half  a  mile.     A  cir- 
eulai  motion  was  distinctly  perceived,  the  water  flying 
off  in  jets  from  the  circumference  of  a  circle,  apparently 
fifty  feet  in  diameter.     A  heavy,  dark  cloud  hung  over 
the  spot,  but  no  descending  tube  appeared,  nor  was  there 
any  progressive  motion.     In  a  short  time  the  cloud  dis- 
persed, and  the  surface  of  the  sea  resumed  its  former 
state. 

154.  It  is  by  no  means  uncommon  for  several  water- 
spouts to  appear  at  the  same  time.     In   May,   18^0. 
Lieutenant  Ogden  beheld,  on  the  edge  of  the  Gulf  stream, 
no  less  than  seven  in  the  course  of  half  an  hour :  vary- 
ing in  their  distance  from  the  ship  from  two  hundred 
yards  to  two  miles. 

155.  DIMENSIONS.     The  diameter  of  the  spout  at  its 
base  ranges  from  a  few  feet  to  several  hundred,  and  its 

Does  the  water-spout  always  undergo  these  changes  1 

Under  what  forms  is  it  sometimes  seen  1 

How  many  have  been  seen  at  once  1 

What  is  the  breadth  and  height  of  water-spouts  1 


72  AERIAL    PHENOMENA. 

altitude  is  supposed  by  some  to  be  at  times  as  great  as 
a  mile.  In  the  account  given  by  the  Hon.  Capt.  Napier, 
of  a  spout  which  be  beheld  in  30°  47'  N.  Lat..  and  62° 
40'  W.  Lon.,  the  diameter  was  judged  to  be  300  feet ; 
and  the  height  of  the  column  to  the  point  where  it  en- 
tered the  hanging  cloud,  was  computed,  from  observa- 
tions taken  by  the  quadrant,  to  be  1720  feet,  or  nearly 
one-third  of  a  mile. 

156.  POPULAR  ERROR.  It  is  a  common  belief,  that 
water  is  drawn  up  by  the  action  of  the  spout  into  the 
clouds  ;  but  there  is  no  proof,  whatever,  of  a  continuous 
column  within  the  whirling  pillar,  and  the  fact,  that  the 
water,  which  sometimes  falls  from  a  spout  upon  the 
deck  of  a  vessel  at  sea  is  always  fresh,  sufficiently  re- 
futes the  idea.  The  torrents  of  rain,  by  which  this  phe- 
nomenon is  often  accompanied,  can  be  fully  accounted 
for  by  the  rapid  condensation  of  vapor  that  occurs,  when 
the  warm,  humid  air  of  the  sea  flows  inward  to  the 
vortex  of  the  whirl,  and  there  combines  with  the  cold 
air  of  the  upper  regions  of  the  atmosphere,  which  de- 
scends to  fill  the  partial  void.  From  this  union  the 
electric  phenomena  of  water-spouts  arise,  and  the  vio- 
lent hail-showers  that  at  times  prevail  ;  the  mode,  how- 
ever, in  which  they  originate,  will  be  explained  here- 
after. 

When  a  vessel  is  in  the  vicinity  of  water-spouts,  can- 
non shots  are  usually  fired  for  the  purpose  of  destroying 
them  ;  lest  the  vessel  should  be  injured  if  a  spout  were 
to  pass  over  it.  It  is  not  improbable  that  such  an  effect 
may  be  produced  when  the  spout  is  either  struck  by  the 
ball's,  or  violently  agitated  by  the  concussion  of  the  air 
arising  from  the  discharges. 

157.  SAND  PILLARS.  Another  form  of  the  whirlwind 
is  exhibited  in  the  pillars  of  ?and,  which  are  not  un fre- 
quently seen  in  the  deserts  of  Africa  and  Peru.  Bruce, 
on  his  journey  to  Abyssinia,  beheld  eleven  vast  columns 
of  sand  of  lofty  height,  moving  over  the  plain  at  (he 

\Vhnt  pojuihr  cir.ir  exists  in  regard  to  this  phenomenon? 
For  what  purpose  are  cannon  discharged  1 
Where  do  sand  pillars  occur? 


WATER-SPOUTS.  73 

same  time  ;  some  with  a  slow  and  majestic  motion,  and 
others  with  great  velocity.  Now,  with  their  summits 
reaching  to  the  clouds,  they  rapidly  approached  the 
terrified  observers,  and,  the  next  moment,  were  borne 
away  by  the  wind  with 'incredible  swiftness. 

Their  tops,  at  times,  were  seen  separated  from  the 
main  pillars,  and  the  latter  were  often  broken  in  two,  as 
if  struck  by  a  cannon  shot :  the  diameter  of  the  largest 
was  about  ten  feet. 

While  Mr.  Adanson  was  crossing  the  river  Gambia,  a 
sand-whirl,  twelve  feet  in  breadth  and  two  hundred  and 
fifty  in  height,  passed  within  forty  yards  of  his  boat. 

158.  The  same  phenomena  are  seen  upon  the  Peru- 
vian coast.     "The  sand,"  says  Dr.  Tschudi,  "rises  in 
columns  from  eighty  to  one  hundred  feet  high,  which 
whirl  about  in  all  directions,  as  if  moved   by  magic. 
Sometimes  they  suddenly  overshadow  the  traveler,  who 
only  escapes  by  rapid  riding." 

159.  BENEFICIAL  EFFECT  OF  WINDS.     The  utility 
of  winds  must  be  evident  to  all.    By  their  aid  vast  oceans 
are  crossed,  and  the  products  of  distant  climes  wafted 
from   shore  to  shore.     Different  nations  are  linked  to- 
gether by  social  and  commercial  ties,  the  blessings  of 
civilization  diffused,  and  the  glad  tidings  from  a  better 
world  borne  to  every  land. 

The  growth  and  decay,  both  of  animal  and  vegetable 
life,  vitiates  the  atmosphere,  and  renders  it  unfit  for 
respiration  ;  but  the  winds  prevent  the  deadly  effects 
that  would  flow  from  this  source,  and  the  air  becomes 
pure  and  salubrious,  from  its  constant  circulation. 

Even  the  fierce  tempest  may  be  a  messenger  of  mercy, 
by  sweeping  from  the  air  the  seeds  of  pestilence  and 
contagion. 

The  advantage  of  winds  in  distributing  moisture  to 
Jie  earth,  will  be  seen  in  the  following  pages. 

Describe  those  seen  by  Bruce  and  Adanson. 

What  does  Dr.  Tschudi  relate? 

What  are  the  advantages  arising  from  winds  1 


PART   III. 

AQUEOUS    PHENOMENA. 


CHAPTER  I. 

OF   RAIN. 

160.  RAIN  is  produced  by  the  rapid  union  of  two  or 
more  volumes  of  humid  air,  differing  considerably  in 
temperature ;  the  several  portions  in  union  being  inca- 
pable of  holding  the  same  amount  of  moisture   that 
each  can  separately  retain. 

Tliis  circumstance  is  the  result  of  the  law,  that  the 
capacity  of  the  air  for  moisture  decreases  at  a  faster  rate 
than  the  temperature. 

161.  This  effect  may  be  thus  illustrated  :  4000  cubic 
inches  of  air,  at  the  temperature  of  86°  Fall.,  can  con- 
tain no  more  than  31^  grains  of  moisture,  and  an  equal 
volume,  at  32°  Fah.,  only  7-f  th  grains.     Now,  if  the  two 
volumes  are  mingled  together,  their  average  temperature 
will  be  59°  Fahrenheit,  and  the  weight  of  moisture  they 
unitedly  possess  will  be  39fth  grains.     But,  at  this  tem- 
perature, 3H  grains  is  all  the  moisture  that  8000  cubic 
inches  of  air  can  possibly  retain  ;  since  the  fast  portion, 
by  its  union  with  the  second,  diminished  its  capacity 
one-half,  while  that,  of  the  latter  was* only  doubled.    The 
excess,  therefore,  of  7-f  grains  will  be  condensed,  and 
descend  in  the  form  of  water. 


What  does  part  third  treat  of? 

What  subject  is  discussed  in  chapter  first  1 

How  is  rain  produced  1 

Give  the  illustration. 


OP    RAIN.  75 

162.  Rain  is  the  result  of  such  combinations  on  an 
extensive  scale,  and  the  quantity  that  falls  at  any  par- 
ticular time  or  place,  depends  upon  the  difference  in  the 
several  temperatures  of  the  combining-  volumes,  and  the 
amount  of  moisture  which  each  separately  possesses. 

163.  Winds  are  the  great   natural  agents  by  which 
such   combinations  are  effected,  and  these  occur  most 
readily,  when  the  currents  of  air  are  shifting  and  vari- 
able.    Constant  winds,  blowing  steadily  from  the  same 
quarter  and  possessing  an  unchanging  temperature,  can 
produce  no  such  admixture,  and  they  are  consequently 
attended  with  dry  weather ;  except  in  the  case  where 
they  strike  the  sloping  sides  of  lofty  mountains,  carrying 
the  warm  air  of  the  sea  and  the  vales  far  up  into  the 
colder  regions  of  the  atmosphere. 

164.  RAIN-GAUGE.    The  quantity  of  rain  that  falls  at 
any  station  during  a  given  time,  is  ascertained  by  means 
of  the  rainrgauge,  an  instrument  which  is  constructed 
in  a  variety  of  ways. 

One  of  the  best  consists  of  a  cylindrical,  copper  vessel, 
furnished  with  a  float ;  the  rain  falling  into  the  vessel 
raises  the  float,  the  stem  of  which  is  so  graduated  that 
an  increase  in  depth,  to  the  extent  of  one-hundredth  of 
an  inch,  can  easily  be  measured. 

The  greatest  annual  depth  occurs  at  San  Luis,  Ma- 
ranham,  2°  30'  S.  Lat. ;  and  Vera  Cruz  ranks  next  in 
this  respect.  At  the  former  place,  280  inches  have  been 
observed  to  fall  in  the  course  of  a  year,  and  at  the  latter 
278  inches. 

165.  DISTRIBUTION  OF  RAIN  IN  LATITUDE.    Since 
the  capacity  of  the  air  for  moisture  increases  with  its 
temperature,  we  should  naturally  infer,  that  the  higher 


What  circumstances  influence  the  amount  of  rain  which  falls  at  any 
place  1 

What  great  natural  agents  effect  these  combinations  1 
What  is  said  of  variable  and  constant  winds  1 
What  is  the  rain-gauge  1 

Where  does  the  greatest  yearly  depth  of  rain  occur  ? 
What  is  the  law  of  distri  mtion  in  respect  to  latitude  1 


76 


AQUEOUS    PHENOMENA. 


the  mean  temperature  of  any  region,  the  greater  would 
be  the  amount  of  rain  which  descends  upon  it. 

This  is  true  as  a  general  rule,  for  the  annual  depth 
of  rain  is  found  to  decrease  with  the  increase  of  latitude, 
as  will  be  seen  from  the  annexed  list  of  seven  localities, 
where  the  rain  has  been  measured. 


North  Latitude. 

Annual  depth  of  Rain  in  inches. 

Grenada,       .     . 

12° 

126 

Care  Francois, 
Calcutta,  .    .     . 

19°  46' 
22°  35' 

120 
81 

Rome,      .    .    . 

41°  54' 

39 

London,    .     .    . 

51°  31' 

25 

St.  Petersburg, 

59  D  56' 

16 

Uleaborg,       .    . 

65°    V 

13.5 

1C6.  EXCEPTIONS.  Although  this  general  relation 
to  latitude  exists,  it  is  by  no  means  to  be  supposed,  that 
the  same  amount  of  rain  descends  yearly  upon  all  re- 
gions lying  within  the  same  parallels  ;  local  causes  will 
have  their  influence,  and  create,  in  many  cases,  extra- 
ordinary departures  from  the  common  rule.  Thus, 
Bombay  and  Vera  Cruz  possess,  nearly,  the  same  position 
in  latitude  ;  but  while  at  the  former  city,  the  annual  depth 
of  rain  varies  from  61  to  112  inches,  that  of  the  latter 
ranges  from  120  to  278  inches. 

This  is  owing  to  the  following  circumstances.  Vera 
Cruz  is  backed  by  a  chain  of  lofty  mountains,  rising  be- 
yond the  limits  of  perpetual  frost,  and  hither  the  hot  and 
humid  tropical  air  is  constantly  driven  by  the  trade 
winds,  as  they  sweep  from  the  sea.  Hence  a  great  and 
sudden  reduction  of  temperature  occurs  amid  these  icy 
regions,  and  ih<>  air,  no  longer  capable  of  absorbing  its 
vast  stores  of  moisture,  precipitates  an  immense  quantity 
of  rain. 

lt>7.  At  Bergen,  in  Norway,  it  has  also  been  found, 
that  in  consequence  of  the  moist  south-west  winds  be- 
ini_r  checked  in  their  course  by  the  mountains, more  than 
*SS  inches  of  rain  descend  in  a  year :  a  quantity  greater 


Illustrate. 

State  the  exceptions  and  the  cause. 


OF   RAIN.  77 

(ban   (hat  which   falls   at   Calcutta    during  the  same 
period. 

168.  DAYS  OF  RAIN.    Though  the  annual  amount  of 
rain  is  greater  in  the  low  than  in  the  high  latitudes,  the 
rainy  days  are  not  so  numerous  ;  as  appears  by  the  fol- 
lowing table,  which  presents  the  average  yearly  num- 
ber, within  *.i)«  latitudes  indicated. 

N.  latitude.  Mean  annual  number  of  rainy  dayi. 

From            12°  to  43°  .  78 

"                43°  "  46°  103 

«                46°  «  50°  134 

50°  "  60°  161 

169.  From  this  circumstance  we  should  readily  con 
elude,  that  the  ordinary  rains  of  the  tropical  climes 
must  be  more  powerful  than  those  of  the  temperate  re- 
gions ;  an  inference  which  coincides  with  observation  ; 
for  it  is  stated  by  Mr.  Scott,  that  at  Bombay,  there  once 
fell,  during  the  first  twelve  days  of  the  wet  season,  thirty- 
two  inches  of  rain,  a  quantity  equal  to  the  'yearly  ave- 
rage of  England. 

170.  This  fact  is  also  shown  by  the  sudden  rise  of 
brooks  and  rivulets  ;  a  remarkable  instance  of  which  is 
related  by  Mr.  Elphinstone,  in  the  account  of  his  mis- 
sion to  Cabul.     It  occurred  between  the  Indus  and  Hy- 
daspes,  and  is  thus  described.     "  On  one  occasion,  th« 
rear-guard,  with  some  of  the  gentlemen  of  the  mission 
were  cut  off  from  the  rest  by  .the  sudden  swelling  of  r> 
brook,  which  had  been  a  foot  deep  when  they  began  it 
cross.     It  came  down  with  surprising  violence,  carrying 
away  some  loaded   camels  that  were   crossing    at  the 
time,  and  rising  about  ten  feet  within  a  minute.     Such 
was  its  force,  that  it  ran  in  waves,  like  the  sea,  and  rose 
against  the  bank  in  a  ridge,  like  the  surf  on  the  coast  of 
Coromandel." 

171.  DISTRIBUTION  IN  ALTITUDE.    The  great  stores 
of  atmospherical  humidity  reside  in  the  inferior  strata 

What  is  the  rule  in  respect  to  days  of  rain  1 
Where  are  rains  most  powerful  ? 


78  AQUEOUS    PHENOMENA. 

of  the  air,  and,  for  this  reason,  less  ram  descends  upon 
lofty  table-lands  and  mountains,  than  upon  regions 
situated  lower  down  in  the  same  latitude. 

Tims,  in  India,  on  the  Malabar  coast,  twelve  degrees 
from  the  equator,  the  annual  depth  of  rain  is  13(3  inches  ; 
while  at  Ootacarnund^  in  the  Nhilgerries,  a  region  lying 
a  short  distance  to  the  east,  in  the  same  latitude,  but 
Sr>00  feet  above  the  ocean,  the  yearly  amount  of  rain  is 
only  63.88  inches.  Likewise,  at  Sante  Fe  de  Bogota, 
.New  Grenada,  a  city  that  enjoys  an  elevation  of  8,800  feet. 
in  the  fourth  degree  of  north  latitude,  the  annual  quan- 
tity of  rain  is  nearly  the  same  as  that  which  falls  in 
Germany,  which  is  about  twenty-one  inches. 

Even  slight  variations  in  altitude  cause  perceptible 
differences  in  the  quantity  of  rain.  At  the  Paris  Observa- 
tory, a  rain-gauge  is  placed  in  the  court,  and  another 
upon  the  terrace,  eighty-nine  feet  above.  The  mean 
annual  depth  of  the  rain  which  fell  in  the  court  for  a 
space  of  ten  years,  was  found  to  be  22.44  inches,  and 
of  that  which  descended  upon  the  terrace  during  the 
same  period,  only  19.68  inches. 

172.  RAIN  UPON  COASTS.  We  have  remarked,  that 
the  air  above  the  ocean  is  always  saturated,  and  that  its 
humidity  decreases  as  we  penetrate  from  the  sea-shore 
into  the  interior  of  a  country.  Conformably  to  this  law, 
other  things  being  equal,  more  rain  descends  upon  tho 
coasts  than  upon  the  central  regions  of  a  country  ;  inas- 
much as  a  less  reduction  of  temperature  will  here  pro- 
duce a  precipitation  of  moisture. 

Besides,  when  the  warm,  humid  air  is  borne  inland  by 
the  winds  from  the  sea,  its  course  is  marked  by  descend- 
ing showers,  and  its  inherent  moisture  decreases  with 
its  progress.  Thus,  on  the  west  coast  of  England,  37 
inches  of  rain  fall  in  the  course  of  a  year ;  while  in  the 
interior,  upon  the  eastern  side,  the  annual  depth  is  25 
inches.  The  maritime  and  inland  regions  of  France 


Give  the  rule  in  regard  to  distribution  in  altitude.    Illustrate. 
Compare  the  rain  upon  coasts  and  inland  regions. 
Why  Is  there  a  difference  1    Give  instances. 


RAINS    WITHIN    THE    TROPICS.  70 

and  Holland  differ,  in  this  respect,  one  inch.  In  this 
country,  the  yearly  average  fall  of  rain  at  Boston,  for  a 
period  of  22  years,  is  39.23  inches  ;  at  Hanover,  New 
Hampshire,  38  inches  ;  in  New  York  State,  36  inches  ; 
and  in  Ohio,  36  inches.  A  diminution  occurring-  as  we 
advance  into  the  interior,  notwithstanding  the  influence 
of  the  great  northern  lakes,  in  the  last  two  instances. 

RAINS    WITHIN   THE    TROPICS. 

173.  Upon  the  ocean,  in  the  region  of  calms,  where 
the  gusts  of  wind  are  ever  changing  their  direction,  tor- 
rents of  rain  frequently  descend.     On   the  land,  in  all 
places  where  the  trade  wind  blows  constantly  seaward, 
no  rain  falls,  and  the  sky  is  always  serene ;  but,  wher- 
ever disturbances  occur  in  this  current  and  the  mon- 
soons prevail,  the  rains  are  periodical,  and  the  year  is 
divided  info  two  seasons,  the  wet  and  the  dry.     These 
are  so  marked  in  their  character,  that  whole  months 
pass  away  without  a  cloud  obscuring  the  sky,  or  miti- 
gating the  fierce  heat  of  the  sun :  then  the  face  of  na- 
ture entirely  changes,  the  heavens  gather  blackness,  the 
rain  comes  down  like  a  deluge,  and   the  parched  earth 
is  refreshed,    for    many   successive   weeks,    by   copious 
showers. 

174.  RAINY  SEASON.    The  rainy  season  commences, 
in  all  the  countries  within  the  tropics,  at  the  shifting  of 
the  monsoons  ;  and  as  this  change  is  dependent  upon  the 
position  of  the  sun,  it  begins  earlier  in  those  regions 
that  lie  near  the  equator,  than  in  those  more  remote. 

At  Panama,  8°  48'  N.  Lat.,  the  rain  falls  early  in  the 
month  of  March;  but  it  seldom  appears  at  St.  Bias, 
California,  before  the  middle  of  June.  In  Africa,  near 
the  line,  the  rainy  season  begins  in  April,  both  upon 
the  sea-coast  and  in  the  interior ;  but  in  the  countries 
watered  by  the  Senegal,  it  commences  in  June,  and  lasts 
till  November. 

How  are  rains  distributed  within  the  tropics  1 

How  is  the  year  divided  where  the  monsoons  prevail  ? 

When  does  the  rainy  season  occur? 

In  what  regions  early  1    In  what  late  1    Illustrate. 


PO  AQUEOUS    PHENOMENA. 

In  India,  the  rains  occur  in  May,  at  the  southern  ex- 
tremity of  the  Malabar  coast,  but  do  not  reach  Delhi 
until  nearly  the  end  of  June. 

175.  CAUSE.     These   stated  rains   originate  in    the 
change  of  the  periodical  winds,  by  which  the  union  of 
vast  volumes  of  air,  differing  in  temperature,  is  rapidly 
effected.     The  subject  cannot  be  better  illustrated,  than 
by  recurring  to  the  origin  of  the  monsoons  of  India.  (Art. 
106.)    Early  in  the  month  of  June,  the  soil  of  the  penin- 
sula becomes  intensely  heated  by  the  vertical  rays  of  the 
sun,  and  powerful  currents  of  rarefied  air  then  ascend 
from  the  earth.     To  supply  the  deficiency  thus  created, 
the  warm  and  humid  atmosphere  of  the  equatorial  seas 
flows  in,  constituting  the  south-west  monsoon  ;  this  wind 
now  mingles  with   the  cool,  dry  air,  which  the  north- 
east monsoon,  for  the  six  previous   months,   has  been 
constantly  bringing   to   the   peninsula   from   the  polar 
and  temperate, dirties,  and  thus  produces  a  combination 
favorable  to  the  precipitation  of  rain,  upon  a  most  exten- 
sive scale. 

176.  PERIODICAL  RAINS  OF  INDIA.    On  the  Malabar 
coast,  the  south-west  monsoon  is  ushered  in  by  terrific 
istorms  of  thunder  and  lightning,  the  water  pours  down 
in  torrents,  and,  when  the  thunder  has  ceased,  nothing 
is  heard  for  several  days  but  the  rush  of  the  descending 
rain,  and  the  roar  of  the  swelling  streams.     In  a  few 
days,  the  storm  ceases,  and  the  earth,  which  before  was 
withered  by  the  glowing  atmosphere,  is  now,  as  if  by 
manic,  suddenly  clothed  with  the  richest  verdure  ;  the 
air  above  lion  is   pure  and  balmy,  and   bright    tropical 
clouds  sail  traii'juilly  through  the  sky. 

After  this,  the  rains  fall  at  intervals  for  the  space  of 
a  month,  when  they  again  return  with  great  violence. 
In  July,  they  attain  their  height,  and  from  that  time 
gradually  subside  until  the  end  of  September,  when  the 
t-casoii  closes,  as  it  began,  in  thunders  and  tempests. 

177.  The  following  table,  the  result  of  the   observa- 


How  do  these  rains  originate  1 
Describe  those  ot  India. 


RAINS    WITHIN    THE    TROPICS.  81 

lions  of  twelve  years,  shows  the  mean  monthly  average 
for  the  rainy  season,  at  Bombay ;  and  serves  to  elucidate 
the  preceding  remarks. 

Inches. 

June,  24 

July,  23.95 

August,         18.87 
Sept.,  14.06 

Oct.,  1.06 

178.  The   south-west  monsoon  does   not,    however, 
bring  rain  to  the  whole  of  India.     Parallel  to  the  icest- . 
ern  coast  runs  a  chain  of  high  mountains,  termed  the 
Ghauts:    here  the   monsoon   is  arrested  in  its  course, 
and  most  of  the  moisture  with  which  it  is  charged,  is 
precipitated,  ere  it  arrives  at  the  central  table-land  of 
Mysore.     On  the  eastern,  or  Coromandel  coast,  its  in- 
fluence is  not  felt,  and  the  seasons  are  here  reversed. 
From  March  till  June,  the  winds  are  hot  and   moist, 
blowing  mostly  from  the  south,  over  the  Bay  of  Ben- 
gal ;  from  June  to  October  the  heat  is  very  great,  but 
about  the  middle  of  the  latter  month,  the  cool,  north- 
east monsoon  commences,  bringing  the  periodical  rains, 
which  terminate  by  the  middle  of  December  ;  the  mon- 
soon continuing  to  blow  until  the  beginning  of  March. 

179.  PERIODICAL  RAINS  OF  CONGO.     We  trace  the 
rainy  and  dry  seasons  of  Congo,  in  the  southern  hemi- 
sphere, to  the  same  cause.    In  general,  from  about  March 
to    September,  no   rain   descends,   but   gales    from   the 
south  and  soutli-east  temper  the  burning  atmosphere. 
In  October,  hot  and  humid  winds  blow  from  the  north- 
west over  the  Gulf  of  Guinea,  and  the  country  is  then 
flooded  by  frequent  rains,  which  continue  to  increase 
until  January.      Slight  showers   then  fall  at  intervals 
until  March,  when  the  rains  recommence  and  continue 
for  a  short  time. 

Illustrate  from  the  table. 

What  is  the  influence  of  the  Ghauts  upon  the  south-west  monsoon? 

What  is  said  of  the  seasons  on  the  eastern  coast  1 

What  wind  brings  the  rains  to  this  region  1 

Describe  the  periodical  rains  of  Con^o. 


82  AdUEOUS    PHENOMENA. 

RAINS  IN  THE  HIGHER  LATITUDES. 

180.  Beyond  the  tropics,  the  rains  no  longer  occur  at 
stated  periods,  but  are  distributed  throughout  the  sea- 
sons without  regard  to  any  law. 

Thus,  in  the  west  of  England,  the  amount  of  rain  in 
winter  is  eight  times  greater  than  in  summer  ;  but  in 
Germany,  it  is  one-half  of  what  falls  in  summer,  and 
at  St.  Petersburg  a  little  more  than  one-third.  In  Italy 
the  greatest  quantity  descends  in  autumn.  There  is  the 
same  irregularity  in  the  number  of  rainy  days;  for  in 
the  west  of  England,  there  are  more  rainy  days  in  win- 
ter than  in  summer ;  but  in  Siberia,  it  rains  four  times 
as  often  in  summer  as  in  winter. 

181.  RAINY  WINDS.     The  rains  in  the  higher  lati- 
tudes, as  well  as  within  the  tropics,  depend  upon   the 
changes. of  the  wind;  though  one  wind  may  be  more 
productive  of  rain  than  another,  and,  in  different  regions, 
the  rainy  winds  do  not  always  blow  from  the  same  di- 
rection. 

In  Europe,  north  of  the  Alps,  the  north-east  wind  is 
dry  and  cold,  since  it  sweeps  over  the  land  from  the 
higher  latitudes ;  but  the  south-west  wind  brings  the 
rain,  for,  coming  over  the  Atlantic  from  southerly 
climes,  it  is  warm  and  humid,  and  its  capacity  for  moist- 
ure is  constantly  decreasing. 

Out  of  one  hundred  showers  that  were  noted  at  Ber- 
lin, scarcely  any  occurred  when  the  north-east  wind  pre- 
vailed ;  while  nearly  half  were  brought  by  the  winds  from 
the  south-west  and  vest.  Moreover,  it  rained  only  once 
for  every  nim  ti*nr«  that  the  easterly  winds  blew,  but 
thrice  for  the  saire  number  of  times  in  which  the  south- 
westerly breezes  predominated. 

182.  The  reverse  of  this  occurs  on  the  eastern  coast 
of  the  United  States,  for  here  the  north-east  winds  give 
rise  to  the  long  storms  of  the  fall  and  spring.     At  these 
seasons,  as  appears  from  the  observations  of  Dr.  Hale, 

Where  are  the  rains  irregular  7    Give  cases. 

What  is  the  rainy  wind  of  Northern  Europe  1 

Why  is  it  rainy  ?    Give  instances. 

Whence  comes  the  rainy  wind,  on  the  eastern  coast  of  the  United  States' 


REGIONS    WITHOUT    RAIN.  83 

of  Boston,  continued  through  a  period  of  twenty-two 
years,  the  winds  are  colder  than  the  atmosphere  of  the 
land,  and  as  they  come  from  the  sea  charged  with  moist- 
ure, the  cause  of  the  rain  is  readily  discerned. 

REGIONS    WITHOUT    RAIN. 

183.  EGYPT.     In  Egypt  it  scarcely  ever  rains.     At 
Cairo,  there  is  an  average  of  four  or  five  showers  a  year ; 
but,  as  we  recede  from  the  coast,  it  becomes  more  rare, 
until  in  Upper  Egypt,  under  the  cloudless  sky  of  Thebes, 
a  man's  life  may  pass  away  without  his  ever  beholding 
a  single  rain. 

184.  The  cause  of  this  scarcity  of  rain  is  to  be  sought 
in  the  peculiar  conformation  of  the  surface  of  this  coun- 
try.    It  is  a  narrow  valley,  bounded  by  two  mountain 
ridges  on   the  east  and  west ;   the   first   prevents   the 
moisture  exhaled  from  the  Red  sea  from  reaching  the 
ralley,  and,  as  the  African  deserts   extend  beyond  the 
western  range,  no  source  of  rain  exists  in  this  quarter. 

185.  The  northerly  winds,  which  blow  from  May  till 
October,  bearing  off  the  vapors  of  the  Mediterranean, 
pass  over  the  whole  length  of  the  valley  of  the  Nile,  with- 
out meeting  any  obstruction  ;  and  it  is  only  when  they 
are  driven  up  the  high  range  of  the  Abyssinian  moun- 
tains, that  they  become  sufficiently  cooled  to  precipitate 
rain.     Here  it  descends  most  copiously  during  the  sum- 
mer months,  swelling  the  tributaries  of  the  Nile,  and 
producing  its  annual  inundation. 

186.  Much  of  the  humidity  brought  by  these  con- 
stant winds,  can    be    retained    by  the   atmosphere  of 
Egypt,  without  being  precipitated  ;  since  it  is  far  below 
the  point  of  saturation,  in  consequence  of  the  prevalence 
of  hot,  dry  winds  from  the  desert,  (Art.  114,)  and  the  ex- 
treme aridity  of  the  soil. 

So  free  from  moisture  is  the  ground,  that  myriads  of 
human  bodies  have  rested  for  centuries  within  its  bosom 


Why  is  it  rainy  1 

What  is  said  of  Egypt  1 

Why  is  it  that  rain  rarely  falls  in  this  country  ? 

What  is  said  of  the  dryness  o(  the  soil  1 


81  AQUEOUS    PHENOMENA. 

without  suffering  the  least  decay  ;  and  in  a  collection  of 
antiquities,  now  in  the  British  Museum,  there  is  an  an- 
cient model  of  an  Egyptian  house,  the  store-rooms  of 
which,  when  first  discovered,  were  full  of  grain  that  had 
remained  uninjured  for  ages. 

187.  PERU.     Along  the  coast  of  Peru  is  stretched  a 
plain  of  sand,  five  hundred  and  forty  leagues  in  length, 
ami  varying  from  three  to  twenty  in  breadth,  upon  which 
no  rain  descends.     So  rare  is  the  occurrence  of  a  real 
shower  at  Lima,  that  it  is  a  source  of  terror  ;  and  when 
such  an  event  happens,  religious  processions  parade  the 
streets,  imploring  the  protection  of  heaven  for  their  en- 
dangered city. 

The  want  of  rain  in  tins  region  is  thus  explained. 
Parallel  to  the  coast  of  Peru,  and  at  a  short  distance 
from  the  sea,  extends  the  lofty  range  of  the  Andes,  the 
peaks  of  which  rise  far  above  the  limit  of  perpetual  frost. 
The  constant  east  wind,  sweeping  from  the  Atlantic 
to  the  Pacific,  across  the  extreme  breadth  of  South 
America,  gradually  ascends  the  slope  of  the  Andes ; 
but  by  the  time  it  has  passed  their  summits  most  of  the 
vapors  with  which  it  is  charged,  are  precipitated,  and  it 
comes  to  the  shores  of  Peru  comparatively  destitute  of 
moisture. 

188.  Moreover,    as   a   sandy   soil   is   naturally   dry, 
scarcely  any  evaporation  occurs,  and  the  hot  air  of  ilie 
plains  possesses  but  little  humidity.    For  these  reasons, 
the  difference  in  the  temperature  of  two  or  more  com- 
bining volumes  of  air  is    rarely  sufficient  to  produce 
rain. 

189.  A  similar  destitution  of  rain  exists  on  the  north- 
west coast  of  Africa,  where  the  desert  of  Zahara  reaches 
the  Atlantic.     In  this  region,  intervals  of  six  or  seven 
y curs  occur  between  the  showers. 

190.  CONSTANT  RAINS.     In  Guiana,  it  rains  for  a 

What  is  said  of  Peru  1 

Is  a  copious  shower  regarded  as  a  blessing  at  Lima  1 

Explain  the  cause  of  this  scarcity  of  rain.  . 

\N  hat  other  region  is  destitute  of  rain  1 


CONSTANT    RAINS.  85 

great  portion  of  the  year ;  nor  is  this  surprising,  when  we 
reflect  that  this  country  is  a  low  and  marshy  region,  over- 
spread with  immense  forests  ;  situated  but  a  few  degrees 
north  of  the  equator,  and  subjected  to  the  influence  of 
the  north-easterly  trade. 

The  fierce  heat  of  the  sun  fills  the  atmosphere  with 
vapor,  which  returns  to  the  earth  again  in  incessant 
showers,  as  the  cool  air  from  the  ocean  flows  in  from  tin1, 
higher  latitudes. 

In  the  interior,  amid  the  primeval  forests,  the  sun 
and  stars  are  seldom  visible,  and  the  rains  not  unfre- 
quently  continue  for  jive  or  six  months,  with  scarcely 
any  intermission. 

191.  According  to  Darwin,  rain  thus  prevails  at  the 
Straits  of  Magellan.    "  At  Port  Famine,"  says  the  writer, 
"  we  have  rounded  mountains,  concealed  by  impervious 
forests,  which  are  drenched  with   rain  brought  by  an 
endless  succession   of  gales :  rock,  ice,  snow,  wind  and 
•water,  all  warring  with  each  other,  here  reign  in   abso- 
lute sovereignty."     It  is  a  proverbial  saying,  in  the  Isle 
of  Chiloe,  43°  S.  Lat.,  that  it  there  rains  six  days  of  the 
week,  and  is  cloudy  on  the  seventh. 

192.  EXCESSIVE   SHOWERS.     The  quantity  of  rain 
that  falls  during  a  single  shower  is  sometimes  amazing. 
At  Cayenne,  Admiral  Roussin  found,  on  one  occasion, 
that  ten   inches  and  three  quarters  fell  in  the  course  of 
ten  hours.     There  fell  at  Genoa,  Oct.  25th,  1822,  thirty 
inches  in  twenty-four  hours ;    and  at  Geneva,  on  the 
20th  of  May,  1827,  six  inches  in  three  hours.     In.  the 
famous  Catskill  storm  of  July  26th,  1819,  a   tub,  very 
nearly  as  large  at  the  bottom  as  at  the  top,  was  filled  to 
the  depth  of  fifteen  inches  and  a  half  in  four  hours. 

193.  RAIN  WITHOUT  CLOUDS.     Singular  as  it  may 
appear,  there  are  yet  many  well-attested  instances  of 
showers  occurring  when  the  sky  was  clear.     This  phe- 
nomenon was  several  times  observed  by  Humboldt ;  and, 

What  is  said  of  the  rains  of  Guiana  ?    What  of  those  at  Port  Famine '? 
Give  instances  of  excessive  showers. 
Does  rain  ever  fall  from  a  cloudless  sky  1 


86  AQUEOUS    PHENOMENA. 

according  to  Kaemtz,  it  happens  in  Germany  twice  or 
thrice  in  a  year.  On  the  9th  of  August,  1837,  a  shower 
fell  at  Geneva,  when  the  sky  was  cloudless,  that  lasted 
two  or  three  minutes  ;  and  at  Constantinople,  rain  was 
seen  to  fall  by  M.  de  Neveu,  for  the  space  of  ten  min- 
utes, when  the  heavens  were  perfectly  serene.  Accord- 
ing to  Le  Gentil,  this  occurrence  is  by  no  means  un- 
common in  the  island  of  Mauritius,  during  the  preva- 
lence of  the  south-east  winds  ;  slight,  showers  falling  in 
the  evening,  when  the  stars  are  shining  brilliantly. 

194.  CAUSE.  The  following  explanation  has  been 
given  of -this  phenomenon.  When  rain  is  produced  by 
the  intermixture  of  different  volumes  of  air,  the  precipi- 
tated moisture  usually  assumes,  at  first,  the  form  of 
small  globules  of  vapor;  an  assemblage  of  which  in  the 
higher  regions  of  the  atmosphere  constitutes  clouds.  As 
the  process  of  condensation  advances,  more  moisture  ig 
precipitated,  and  the  globules  uniting  in  rain-drops,  de- 
scend to  the  earth.  Now  it  is  supposed,  that,  at  times, 
the  humidity  of  the  atmosphere  is  condensed  at  once 
into  rain,  without  passing  through  the  intermediate  state 
of  cloud  ;  and  under  these  circumstances  a  shower  might 
'all  from  a  cloudless  sky. 


CHAPTER  II. 

OP   FOGS. 

195.  Fogs,  or  mists,  are  visible  vapors,  that  float  in 
the  atmosphere,  near  the  surface  of  the  earth. 

They  originate  in  the  same  causes  as  rain  ;  viz.,  the 
union  of  a  cool  body  of  air  with  one  that  is  warm  and 
humid  ;  when  the  precipitation  of  moisture  is  slight, 
fogs  are  produced  ;  when  it  is  copious,  rains  are  the 
result. 


Give  cases.    How  is  this  circumstance  explained  ? 
What  is  the  subject  of  chapter  second7?    Define  fogs. 
In  what  do  they  originate  ? 


DISTRIBUTION    IN    LATITUDE.  87 

196.  CONSTITUTION.     When  a  mist  is  closely  exam- 
ined, it  is  found  to  consist  of  minute  globules,  and  the 
investigations  of  Saussure,  and  Kratzenstein,  lead  us  to 
suppose,  that  they  are  hollow  ;  for  the  latter  philosopher 
discovered  upon  them  rings  of  prismatic  colors,  like  those 
seen  upon  soap  bubbles ;  (C.  79,)  and  these  could  not 
exist  if  the  globule  was  a  drop   of  water,  with   no  air 
or  gas  within.     The  size  of  these  globules  is  greatest 
when  the  atmosphere  is  very  humid,  and  least  when  it 
is  dry. 

DISTRIBUTION    IN   LATITUDE. 

197.  TROPICAL    REGIONS.     Fogs  are  not  generally 
common  in  the  equatorial  clime,  its  high   mean  temper- 
ature being  favorable  to  the  dissolution  of  vapor.    They 
are  however,  by  no  means,  unfrequent  at  certain  seasons, 
and  in  particular  localities. 

Thus,  in  India,  just  before  the  commencement  and  at 
the  close  of  the  rainy  season,  when  the  air  contains  an 
excess  of  moisture,  but  not  enough  to  produce  rain, 
clouds  of  mist  so  dense  and  thick  obscure  the  atmos- 
phere, that  they  are  not  dissipated  until  late  in  the  morn- 
ing. 

During  the  month  of  December,  the  towering  sum- 
mits of  the  Abyssinian  mountains  are  also  shrouded  in 
impenetrable  fogs.  Peru  is  remarkable  for  its  misty 
atmosphere,  of  which  we  shall  soon  speak  more  par- 
ticularly. 

198.  TEMPERATE  REGIONS.    In  the  temperate  climes, 
mists  frequently  occur  ;  but  are  of  comparatively  small 
extent. 

199.  POLAR   REGIONS.     In  the   polar   regions  they 
spread  far  and  wide,  over  sea  and  land,  and  prevail  both 
in  winter  and  summer. 

At  the  beginning  of  winter,  the  whole  surface  of  the 
northern  ocean  steams  with  vapor,  denominated  frost 
smoke;  but  as  the  season  advances,  and  the  .cold  in- 

What  does  a  mist  consist  of  1 

Where  do  fogs  prevail  least  ?    When  do  they  appear  in  India  1 

Where  do  they  occur  frequently  ?    Where  most  "i 


88  AaUEOUS    PHENOMENA 

creases,  it  disappears.  Towards  the  end  of  Tune,  when 
the  summer  commences,  the  fogs  are  again  seen,  mant- 
ling the  land  and  sea  with  their  heavy  folds.  By  the 
middle  of  summer,  these  also  disappear,  to  return  again 
at  the  approach  of  winter. 

So  dense  are  these  mists,  that  they  render  the  naviga 
lion  of  the  polar  seas  extremely  dangerous,  and  the  nar 
ratives  of  the  hardy  explorers  of  these  inhospitable 
climes  are  full  of  the  perils  arising  from  this  source. 
Simpson,  who  penetrated  by  land  to  the  Arctic  ocean, 
in  1837,  speaks  of  the  dense"  fog  that  often  involved  his 
party  in  imminent  danger,  while  coasting  along  these 
ice-bound  shores. 

200.  CAUSE.     The  phenomena  of  the  polar  fogs  are 
explained  in  the  following  manner.     During  the  short 
Arctic   summer,  the  earth   rises   in   temperature    with 
much  greater  rapidity  than  the  sea  :  the  thermometer 
sometimes  standing,  according   to  Simpson,  at  71°  Fall. 
in  the  shade,  while  ice  of  immense   thickness  lines  the 
shore.    Flowers  also  bloom  at  the  surface  of  the  ground, 
when  the  soil  is  firmly  frozen  four  inches  below.     The 
air,  incumbent  upon  the  land   and  water,  partakes  of 
their  respective  temperatures  ;    and  on  account  of  the 
ceaseless  agitations  of  the  atmosphere,  a  union  of  the 
warm  air  of  the  ground  with  the  cool  air  of  the  ocean 
will  necessarily  occur,  giving  rise  to  the  summer  fogs. 
But,  as  the  winter  approaches,  the  land  becomes  colder 
than  the  sea  ;  since  the  heat  acquired  during  the  sea- 
son of  summer  is  lost  far  more  slowly  by  the  latter  than 
by  the  former ;  and  then,  upon  the  warm  surface  of  the 
ocean,  will  lloat  the  frost  smoke,  as  the  cool  air  flows 
down  upon  it  from  the  adjacent  shores. 

LOCAL   DISTRIBUTION. 

201.  Fogs  are  found  along  the  course  of  rivers,  upon 
.the  sides  of  mountains,  and  over  shoals  and  capes.     It 
is  not  difficult  to  detect  the  cause  of  their  appearance  in 
these  situations. 

Describe  the  polar  fogs.     Explain  the  cause  of  their  formation. 
In  what  localities  are  mists  found  1 


LOCAL    DISTRIBUTION.  89 

202.  RIVERS.    The  banks  of  a  river,  during  the  night, 
lose  more  heat  by  radiation  than  the  stieam  itself,  and 
to  the  air,  which  rests  upon  each,  a  similar  difference  in 
temperature  is  imparted.     By  the  fluctuations  of  the  at- 
mosphere, an  intermixture  is  readily  effected  ;  and  the 
superfluous  moisture  is  seen,  in  the  morning,  floating  in 
fog  over  either  bank,  and  tracing  in  a  wreath  of  mist 
the  devious  windings  of  the  stream.     Fogs  usually  oc- 
cur over  rivers  in  the  early  part  of  the  day  ;  for  the  rea- 
son, that  soon   after  the  sun  rises  the  equality  of   tem- 
perature is  restored,  and  the  vapor  is  then  rapidly  dissi- 
pated. 

203.  When  Sir  Humphrey  Davy  descended  the  Dan- 
ube   in    1818,    he    observed    that   mist   was   regularly 
formed,  when  the  temperature  of  the  air  on  shore  was 
from  three  to  six  degrees  lower  than  that  of  the  stream  ; 
and,  at  the  junction  of  the  Inn  and  Ilz  with  the  Danube, 
at  six  o'clock  on  a  morning  in  June,  he  found  the  distri- 
bution of  temperature,  and  the  state  of  the  mist,  to  be  as 
follows. 


Temperature  of  the  air 
over  the  land. 

Temperature  of  the 
rivers. 

State  of  the  atmosphere 
over  rhe  livers. 

54°  Fah.        5 

Danube,  62°  Pah. 
Inn.          56° 
Ilz,           55° 

Thick  fog  all  over. 
Dense  mist  all  over. 
Light  mist. 

204.  It  is  not  essential  to  the  production  of  fogs,  that 
the  air  of  the  stream  should  be  warmer  than  that  of  the 
land  ;  it  may  be  colder,  arid  then  fogs  appear,  if  the  dif- 
ference of  temperature  is  sufficiently  great.  This  is  the 
case  on  the  Mississippi.  During  the  spring  and  fall, 
mists  form  over  the  river  in  the  day  time,  when  the  tem- 
perature of  the  water  is  several  degrees  below  that  of 
the  air  above,  and  the  air  above  cooler  than  the  atmos- 
phere upon  the  banks.  These  diurnal  fogs,  though 
often  extremely  dense,  are  chiefly  confined  to  the  river, 
and  seldom  extend  beyond  its  banks. 

Why  do  they  occur  along  the  course  of  rivers  1 
.  State  Sir  H.  Davy's  observations. 
Under  what  other  circumstances  can  mists  occur  ?    Give 


90  AQUEOUS    PHENOMENA. 

205.  On  the  31st  of  Dec.   1847,  as  the  writer  was 
standing  upon  a  bridge,  which  crosses  one  of  the  tribu- 
taries of  the  Connecticut,  he  was  unable  to  perceive  a 
mill,  140  yards  distant,  in  consequence  of  the  dense  fog 
which  covered  the  river.     Upon  examination,  the  tem- 
perature of  the  water  was  found  to  be  32°  Fah.,  and  that 
of  the  air,  close  by  the  stream,  40°  Fah. :  a  difference, 
here  existing,  of  fourteen  degrees. 

206.  MOUNTAINS.    Fogs  appear  upon  mountains,  be- 
cause the  warmth  of  the  atmosphere  diminishes  as  we 
ascend,  (Art.  51,)  and  the  cool  and  shady  forests,  that 
clothe  their  sides,  contribute   still  further  to  lower  the 
temperature.     Hence,  when  the  warm  air  of  the  vales 
is  gradually  driven  up  by  the  wind  into  these  regions,  its 
capacity  for  moisture  is  continually  reduced,   until  ai 
length  a  precipitation  occurs,  and  clouds  of  mist  involve 
both  cliff  and  forest. 

207.  At  the  Mountain-House,  on  the  Catskill  range, 
the  temperature  in  summer  is  ten  degrees  lower  than  in 
the  valley  of  the  Hudson  :  and  often  when  a  breeze  sets 
towards  the  mountain,  a  spectator  upon  the  summit  be- 
holds, at  first,  a  wreath  of  mist  extending  along  the 
base ;  soon  the  lower  belt  of  forest  is  concealed   from 
view,  and  the  fog  continuing  to  ascend,  thickening  and 
spreading  on  every  side,  the  landscape  ere  long  is  com- 
pletely veiled,  and  a  chilling  wind  sweeps  past,  loaded 
with  moisture.     A  fact  related  by  Sir  John  Herschel, 
affords  a  striking  illustration  of  the  iniluence  of  trees  in 
condensing  moisture.    During  the  residence  of  this  gen- 
tleman at  the  Cape  of  Good  Hope,  he  observed,  that  on 
the  side  of  the  Table-Mountain  from  which  the  wind 
blew,  the  clouds  were  spread  out  and  descended  very  low, 
and  often  without  any  rain  falling  ;  while  on  the  oppo- 
site side  they  poured  over  the  face  of  the  mountain  in 
dense  masses  of  vapor.    Sir  John  discovered,  when  walk- 
ing beneath  tall  fir  trees,  while  these  clouds  were  closely 
overhead,  that  he  was  subjected  to  a  copious  shower,  but 
on  coming  from  beneath  the  trees,  the  rain  ceased.     On 

Why  do  fogs  appear  upon  mountains  7    Illustrate. 
What  fuel  is  related  by  Sir  John  Horschel } 


LOCAL    DISTRIBUTION.  91 

investigating  the  cause,  he  found  that  the  clouds  were 
condensed  into  rain  on  the  cool  tops  of  the  trees. 

208.  CAPES.     The  reason   for  the  existence  of  fogg 
over  capes  and  headlands   has  already  been  given,  in 
accounting   for   the   prevalence    of  mist   in   the   polar 
climes. 

The  soil  of  these  places  becomes  warmer  in  summer 
than  the  ocean  that  washes  their  shores  ;  but  in  the 
winter,  colder;  and  the  difference  in  temperature  is 
usually  sufficient  to  produce  a  constant  succession  of 
mists. 

209.  SHOALS.      A   similar   state  of  the  atmosphere 
occurs  over  shoals,  inasmuch  as  their  waters  are  colder 
than  those  of  the  main  ocean.     Thus,  Humboldt  found 
near  Corunna,  that  while  the  temperature  of  the  water 
on  the  shoals  was  54°  Fah.,  that  of  the  deep  sea  was  as 
high  as  59°  Fah. 

Under  these  circumstances,  an  intermixture  of  the 
adjacent  volumes  of  air,  resting  upon  the  waters  thus 
differing  in  temperature,  will  naturally  occasion  fogs. 

210.  NEWFOUNDLAND.    Mists  of  great  extent  shroud 
the  sea  on  the  banks  of  Newfoundland,  and  particularly 
near  the  current  of  the  Gulf  Stream.     The  difference 
in  the  warmth  of  the  waters  of  the  stream,  the  ocean, 
and  the  banks,  fully  explains  this  phenomenon.     This 
current,  flowing  from  the  equatorial  regions,  possesses 
a  temperature  5i°  Fah.  above  that  of  the  adjacent  ocean, 
and  the  waters  of  the  latter  are  from  16°  to  18°  warmer 
than  those  of  the  banks.     The  difference,  in  tempera- 
ture, between  the  waters  of  the  stream  and  banks,  has 
even  risen  as  high  as  thirty  degrees. 

211.  ENGLAND.     England,  surrounded   by  a  warm 
sea,  is  subject  to  thick  fogs,  that  prevail  extensively  in 
the  winter.     In  London   they  are   often  so  extremely 
dense,  that  it  is  necessary  to  light  the  gas  in  the  streets 
and  houses  m  the  middle  of  the  day.     On  the  24th  of 

Why  over  capes  1    Why  over  shoals  ? 

How  are  the  fogs  of  Newfoundland  explained  1 

Describe  those  of  England. 


92  AQUEOUS    PHENOMENA. 

February.  J832,  people  in  the  streets  were  unable,  at 
mid-day,  to  see  distinctly  on  account  of  the  fog ;  and  in 
the  evening,  the  city  having  been  illuminated,  as  this 
day  was  the  birth-day  of  the  queen,  boys  went  about 
with  torches,  saying,  "that  they  were  looking  for  the 
illumination."  Similar  fogs  have  been  observed  at  Paris, 
and  Amsterdam.  The  smoke,  arising  from  the  exten- 
sive combustion  in  such  large  cities,  is  regarded,  by  some, 
as  contributing  to  the  density  of  these  extraordinary 
fogs. 

212.  GARUAS.     We  have  seen,  (Art.  187,)  that  along 
the  coast  of  Peru,  the  atmosphere  scarcely  ever  possesses 
sufficient  moisture* to  produce  rain  ;    it  contains,  how- 
ever, enough  to  create  widely  extended  and  continued 
fogs. 

The  wintry  season,  in  this  country,  lasts  from  April 
to  October,  and,  throughout  the  whole  of  this  period,  a 
veil  of  mist  shrouds  sea  and  shore.  At  the  beginning 
and  end  of  this  season,  it  rises  between  nine  and  ten  in 
the  morning,  and  disappears  about  three  in  the  after- 
noon, at  the  hottest  portion  of  the  day.  But,  during 
the  months  of  August  and  September,  the  vapor  is  ex- 
tremely dense,  and  rests  for  weeks  immovably  upon  the 
earth.  In  October  and  November,  the  misty  canopy 
begins  to  rise,  and  gradually  growing  thinner,  at  length 
yields  to  the  piercing  rays  of  the  sun,  and  is  entirely 
dissipated. 

213.  These  fogs,  termed  by  the  natives,  Garuas,  are 
said  to  be  at  times  so  heavy,  that  the  moisture  falls  to 
the  earth  in  large  drops,  which  are  formed  by  the  union 
of  small  globules  of  mist.     There  is,  however,  this  dis- 
tinction, between  them  and  rain-drops ;  that  the  latter 
descend  from  the  more  elevated  regions  of  the  atmos- 
phere, while  the  garuas  do  riot  extend    higher  than 
hn-ln-  hundred  feet ;  their  average  altitude  varying  from 
seven  to  eight  hundred. 

214.  Passing  eastward  from  the  Puna  table-lands  of 
Porn,  across  the  lofty  ridges  of  the  Andes,  the  traveler; 

Describe  the  G;iruas. 

Describe  the  state  of  the  atmosphere  east  of  the  Puna  regions. 


LOCAL    DISTRIBUTION.  93 

after. descending  a  few  hundred  feet,  arrives  at  a  region 
totally  different  from  that  which  he  has  just  left.  He 
no  longer  breathes  a  pure  and  refreshing  atmosphere  ; 
for  the  air  is  loaded  with  vapors,  and  the  wooded  ranges, 
railed  the  Ceja  de  la  Montana,  or  mist  of  the  mountain, 
are  clouded  with  fogs  throughout  the  year.  In  the 
dry  season,  these  are  dissolved,  during  the  day,  by  the 
powerful  influence  of  the  sun  ;  but  in  the  winter  they 
condense  upon  the  hills,  and  descend  in  ceaseless  tor- 
rents of  rain. 

215.  Proceeding  in  the  same  direction,  from  the  Ceja 
de  la  Montana,  the  magnificent  slope  of  the  Andes  soon 
opens  upon  the  sight ;  not  reposing  beneath  a  clear  and 
azure  sky,  but  overshadowed  by  a  thick  veil  of  mist, 
impenetrable  to  the  rays  of  the  early  sun,  and  yielding 
only  to  his  noontide  beams. 

216.  The  explanation  of  these  phenomena  is  to  be 
'found,  in  the  constant  advance  of  the  humid  trade  wind, 
from  the  eastern  shores  of  South  America  to  the  tow- 
ering summits  of  the  Andes.     Rising  continually  in  its 
onward  progress  into  higher  and  colder  regions,  its  ca- 
pacity for  moisture  is  ever  diminishing,  and  the  atmos- 
phere is  always  near  the  point  of  saturation.    Its  inland 
course  will  thus  be  marked  by  abundant  rains,  and  when 
these  abate,  fogs  and  mists  succeed  in  their  turn. 

By  the  time  this  great  aerial  current  has  arrived  at 
the  more  elevated  ridges,  most  of  its  humidity  has  been 
discharged  ;  during  the  dry  season  enough  only  remains 
to  produce  extensive  mists  ;  and  when,  at  length,  it 
has  reached  Peru,  it  possesses  scarcely  any  moisture. 
(Art.  187.) 

217.  A  powerful  auxiliary  cause  exists,  in  the  rich 
and  luxuriant  vegetation,  that  springs  up  every  where 
throughout  this  boundless  region.     The  light  of  a  trop- 
ical day,  in   its  meridian  splendor,  can  scarcely  pierce 
the  massive  foliage  of  those  mighty  forests,  which  stretch 
away  for  leagues  from  the  base  of  the  lower  Andes  | 
v\  hile  the  lighter  forms  of  vegetation,  spreading  in  wild 

Explain  the  cause. 

What  is  the  effect  of  the  vegetation  in  this  particular  ? 


94  AQUEOUS    PHENOMENA. 

exuberance  over  the  higher  belts,  effectually  shield  tie 
earth  from  the  fierce  rays  of  the  sun,  and  check  the  pro- 
gress of  evaporation.  The  soil,  thus  shaded,  is  always 
moist,  and  the  air  warm  and  humid  ;  and  from  the 
causes  already  stated,  such  results  are  here  produced  as 
we  should  readily  infer — excessive  rains  in  the  lower 
forests  (190),  and  clouds  of  mist  upon  the  more  elevated 
ranges. 


CHAPTER  III. 

OF   CLOUDS. 

218.  The  name  of  clouds  is  given  to  those  collections 
of  vapor,  that  float  at  a  lofty  altitude  above  the  earth. 

219.  Though  differing  from  fogs  in  situation,  they 
originate  in  precisely  the  same  causes ;  being  formed, 
in  the  higher  regions  of  the  atmosphere,  by  the  union  of 
warm   and  cold  air,  when   the  combining  volumes  are 
over  saturated.     The  excess  of  humidity,  when  slight, 
then  appearing  in  the  atmosphere  in  the  form  of  clouds. 

220.  During  the  daily  process  of  evaporation,  warm, 
humid  currents  of  air  are  continually  ascending  from 
the  earth;  the  higher  they  ascend,  the  colder  is  the  at- 
mosphere into  which  they  enter  ;  and,  as  they  continue 
to  rise,  a  point  at   length  will   be  attained,  where,  in 
union  with  the  colder  air,  their  original  humidity  can  no 
longer  be  retained;  a  cloud  will  then  appear,  which  in- 
creases in  bulk  with  the  upward  progress  of  the  current 
into  colder  regions. 

If  the  cloud  however,  in  its  ascent,  either  meets  with 
a  warmer  stratum  of  air,  or  descends  towards  the  earth 
into  a  region  of  a  higher  temperature,  a  portion  of  the 

What  is  the  subject  of  chapter  third  1 

Define  clouds. 

How  do  they  originate  1 

In  what  manner  do  ascending  currents  produte  clouds? 


OF    CLOUDS.  95 

minute  globules  of  water  which  compose  it.  perhaps  all, 
will  be  re-dissolved,  and  the  cloud  will  either  contract  in 
size,  or  completely  vanish,  according  to  the  increase  of 
heat  to  which  it  is  subjected. 

221.  The  entire  atmosphere,  to  the  altitude  of  many 
thousand  feet,  is  constantly  traversed  by  numerous  hori- 
zontal currents  of  air,  flowing  in  different  directions, 
and  at  different  heights.    Combinations  of  vast  volumes 
of  air,  varying  in  temperature,  must  therefore  at  times 
inevitably  occur,  as  well  in  the  higher  as  in  the  lower 
regions  of  the   atmosphere  ;    and  when  the  excess  of 
moisture  resulting  from  this  union  is  but  small,  clouds, 
with  their  ever-changing  forms,  obscure  the  serenity  of 
the  sky. 

222.  When  Clayton,  on  the  31st  of  July,  1837,  as- 
cended in  a  balloon  from  Louisville,  Ky.,  the  direction 
of  his  course  was  altered  no  less  than  Jive  times,  in  the 
space  of  fourteen  hours.     Once,  when  at  a  very  great 
height,  he  beheld,  a  mile  below  him,  a  snow-white  cloud 
of  a  mountain  shape  drifting  in  an  opposite  direction  to 
that  in  which  he  was  traveling. 

223.  The  upward  impulse  given  to  the  warm  atmos- 
phere near  the  earth,  when  driven  by  the  wind  against 
.he  sloping  sides  of  mountains,  is  also  a  fruitful  source 
of  clouds.    (230.) 

224.  STRATA  OF  CLOUDS.    When  an  extended  range 
of  clouds  settles  down  towards  the  earth,  its  under  sur- 
face often  copies  the  outline  of  the  landscape  immediately 
beneath,  assuming  a  horizontal  direction.    This  is  owing 
to  the  high  temperature  of  the  air  below  the  cloud,  in 
consequence  of  which  the  latter  would  cease  to  be  visi- 
ble, were  it  to  descend  lower  ;  for  the  globules  of  vapor 
would  then  be  dissolved  by  the  warm  atmosphere. 

225.  Above  the  first  range  of  clouds,  the  temperature 
is  often  much  higher  than  in  the  region  of  vapors  be- 

What  is  said  as  to  the  existence  of  horizontal  currents,  and  their  effects  7 

Relate  the  illustration. 

What  is  the  influence  of  mountains  ? 

What  is  said  as  regards  the  figure  of  the  under  surface  of  clouds  ? 

How  is  the  existence  of  successive  ranges  of  clouds  explained1? 


96  AQUEOUS    PHENOMENA. 

neath.  Here  'the  air  will  be  clear,  and  a  tract  of  con- 
siderable thickness  frequently  intervenes  before  \ve  ar- 
rive at  a  second  range  of  clouds  ;  to  this  may  succeed 
another  body  of  pure  air,  and  still  higher  a  third  range 
of  cloud?,  and  so  on,  alternately. 

226.  The  following  account,  given  by  Jolliffe,  of  his 
aorial  voyage,  which  took  place  in  England,  in  1826, 
is  instructive  in  this  connection. 

"Our  progress,  during  the  first  quarter  of  a  mile,  was 
so  gradual,  as  to  be  nearly  imperceptible ;  but  on  dis- 
charging a  portion  of  the  ballast,  the  balloon  ascended 
with  a  rapidity,  which,  in  a  few  minutes,  buried  us  in 
the  vapors  of  a  dense  mass  of  clouds.  The  temperature 
was  here  cold  and  raw  ;  such  as  I  have  felt  on  a  moun- 
tain-top, when  enveloped  in  fog.  We  loitered  here  for 
some  time  ;  but  at  length  rose  with  uncontrollable  velo 
city,  and  burst,  almost  suddenly,  out  of  this  dark  barrier 
into  realms  of  light  and  glory.  The  stratum  of  clouds 
from  which  we,  had  emerged,  seemed  depressed  to  a  vast 
distance  below  us,  involved  in  radiant  folds,  which  com- 
pletely shut  out  all  view  of  the  earth/' 

227.  THICKNESS.     The  thickness  of  clouds  is  some- 
times immense.    On  the  29th  of  Sept.  1826,  Peytier  and 
Hossard,  two   French  engineers,  were  upon  the  Pyre- 
nees, and   so  stationed,  that  they  beheld,  at   the  same 
time,  the  upper  and  lower  surfaces  of  the  same  cloud. 
As  the  altitude  of  each  station  was  known,  the  thick- 
ness of  the  cloud  was  readily  determined,  and  found  to 
be  1,476  feet.     On  the  succeeding  day,  the  thickness  of 
the  clouds  was  2,788  feet ;  or  more  than  half  a  mile. 

228.  HEIGHT.     The  height  of  clouds  has  been  vari- 
ously estimated.     According  to  observations  given  by 
DattOD,  two-f/Uis  of  all  the  clouds  observed  in  England 
for  the  space  of  five  years,  were  more  than  3,150  feet 
above  the  earth.     By  noting  when  the  upper  and  lower 
surfaces  of  the  clouds  touched  the  peaks  of  the  Pyre- 


R-'atc  the  account  given  by  JollilFe. 

What  is  said  respecting  the  thickness  of  clouds'? 

What  of  their  height  7 


OF    CLOUDS.  9T 

nees,  which  had  previously  been  measured,  Peytier  and 
Hossard  obtained  no  less  lhan  forty-eight  altitudes.  It 
was  thus  found,  that  the  low-r  surfaces  here  varied  in 
height  from  1,476  feet  to  b,200,  and  the  upper  from 
2,952  feet  to  9,840. 

229.  The  computations  of  many  distinguished  ob- 
servers have  been  collected  byKaemt.z;  and  from  these 
it  appears,  that  clouds  range,  in  height,  from  1.300  feet 
to  21,320. 

The  extreme  elevation  here  given  is,  however,  not 
sufficiently  great ;  for  clouds  are  sometimes  seen  float- 
ing above  the  summit  of  Chi mborazo,  which  rises  21,480 
feet  above  the  sea-level ;  and  when  Gay  Lussac,  in  the 
month  of  September,  1804,  ascended  in  a  balloon  to  the 
altitude  of  23,000  feet,  he  beheld  clouds  still  soaring 
above  him,  apparently  at  a  great  height. 

30.  CLOUDS  ON  MOUNTAINS.  When  a  mountain 
range  is  viewed  from  a  distance,  the  various  peaks  are 
frequently  seen  capped  with  a  cloud  ;  while  the  atmos- 
phere between  them  is  perfectly  clear.  This  appear- 
ance sometimes  continues  for  hours,  and  even  en  lire 
days;  and  was  often  noticed  amid  the  Alps  by  the  cele- 
brated Saussure.  It  is  caused  by  the  wind  impelling 
up  the  sides  of  the  peaks  the  warm,  humid  air  of  the 
vales,  which,  in  its  ascent,  gradually  sinks  in  capacity, 
until  it  is  over-saturated,  when  the  excess  of  moisture 
becomes  visible,  and  appears  as  a  cloud. 

231.  This  phenomenon  is  illustrated  by  figure  12. 
Let  ABC  represent  the  outline  of  a  mountain  peak,  up 
the  sides  of  which  a  warm  current  iiows,  in  the  direc- 
tion of  (he  arrows.  Above  the  line  D  E,  the  tempera- 
ture is  below  the  dew-point  of  the  current,  and  its  hu- 
midity is  condensed  into  a  cloud  at  B.  As  the  wind 
sweeps  over  the  summit,  the  cloud  B  i-s  carried  below 
the  line  D  E,  on  the  opposite  sLle,  knd  re-dissolved  in 
the  warm  atmosphere  beneath  ;  but  its  place,  mean- 
while, is  occupied  by  a  fresh  cloud,  caused  by  the  ascent 


What  is  the  appearance  sometimes  presented  by  distant  mo'.uitains  1 
How  is  this  accounted  for  1 

5 


Qg  AQUEOUS    PHENOMENA. 

Fig.  12. 


A  CLOtTD    UPON   A    MOUNTAIN   PEAK.. 


of  the  warm  air  on  the  side  A  B.  It  thus  occurs,  that 
though  the  cloud  upon  the  mountain  is  stationary  for 
hours  together,  yet  the  particles  which  compose  it  are 
continually  clia'iiglng. 

232.  The  appearances  just  described  are  finely  dis- 
played upon  the  St.  Gothard,  a  mountain  in    Switzer- 
land, about  6000   feet  high.     Dark,  heavy  clouds  that 
have  formed  on  one  side  of  the  mountain,  are  frequently 
seen,  passing  rapidly  over  its  summit,  and  descending 
in  dense  masses  into  the  vale  of  Tremola,  on  the  oppo- 
site side  ;  but,  instead  of  filling  the  plains  beneath  with 
thick  vapor,  the  clouds  are  dissolved  by  the  warm  air  into 
which  they  are  precipitated. 

233.  A  singular  instance  of  the  alternate  appearance 
and  disappearance  of  a  cloud  occurred,  not  long  since, 
upon  the  coast  of  England.     A  cloud  wras  seen,  borne 
along  by  the  wind,  apparently  pas^in^  from  one  side  of 
an  arm  of  the  sea  to  the  other,  but  not  extending  across 
the  water.     It  was  visible  over  the  land,  on  each  shore, 
but  the  sky  above  the  water  was  perfectly  serene.    This 
phenomenon  may  be  thus  explained.     Over  the  land,  iu 
the  region  of  the  cloud,  the  air  \vas  below  the  dew-point  ; 
but  over  the  water,  the  sea  being  warmer  than  the  land, 


Explain  from  the  figure. 
Give  the  illustrations. 


CLASSIFICATION.  99 

the  temperature  of  the  air  was  higher,  and  above  the 
dew-point.  When,  therefore,  the  wind  carried  the  cloud 
over  the  sea  it  vanished,  its  moisture  being  re-dissolved 
by  the  atmosphere  ;  but  when  the  body  of 'air  in  which 
the  cloud  had  previously  existed,  arrived  at  the  opposite 
shore,  a  second  precipitation  of  moisture  took  place,  and 
the  cloud  reappeared.  ^ 

CLASSIFICATION. 

234.  Clouds  have  been  divided  into  seven  kinds  ;  three 
original;  viz.  the  cirrus,  the  cumulus,  the  stratus  ;  and 
four  formed  by  combination,  viz.  the  cirro-cumulus,  the 
cirro-stratus,  the  cumulo-stratus,  and  the  nimbus. 

235.  CIRRUS   OR   CURL   CLOUD.      This  cloud  is  so 
called,  from  the  Latin  word  cirrus  or  curl,  because  it 
usually  resembles  a  distended  lock  of  hair.     It  is  dis- 
tinguished from  the  other  kinds  by  its  fibrous  structure, 
the  lightness  of  its  appearance,  and  the  variety  of 'figures 
it  is  capable  of  assuming.    After  a  period  of  fine  weather, 
slender   filaments   of    the   cirrus   are   frequently   seen, 
stretching  like  white  lines  across  the  azure  sky.    Some- 
times these  threads  of  clouds  are  arranged  in  parallel 
bands,  which  in  the   northern    hemisphere,   (wherever 
observations  have  been  taken,)  are  either  directed  from 
south  to  north,  or  from  south-west  to  north-east ;  at  other 
times  they  separate,  resembling  the  tail  of  a  horse ;   a 
form  which  is  known  in  Germany  by  the  name  of  wind- 
trees.     These  filaments  are  also  not  unfrequently  seen 
crossing  each  other,  and  investing  the  sky  with  a  deli- 
cate net-work  of  gauze-like  vapor.     One  of  the  most 
beautiful  forms  of  the  cirrus  occurs,  when  the  fibres  curl 
from  each  side  of  a  band  of  light  cloud,  and  the  whole 
appears  like   the  feathered  grain  of  a  rich  piece  of  ma- 
hogany, (figure  13,  a.) 

236.  The  white  color  of  the  cirrus  renders  it  difficult, 
in  all  cases,  to  detect  its  peculiar  structure  ;  for  the  eye 

Into  how  many  classes  are  clouds  divided? 
What  are  they  ? 
Describe  the  cirrus. 


100  AQUEOUS    PHENOMENA. 

is  dazzled  by  its  excessive  light.  The  cloud  may,  how- 
ever, be  viewed  at  leisure,  by  reflection  from  a  blackened 
mirror,  which  diminishes  the  brightness. 

237.  The  cirrus  soars  the  highest  of  all  clouds.     Its 
altitude,  at  Halle,  in  Germany,  has  frequently  been  esti- 
mated, by  Kaemtz  to  be  not  less  than  21,300  feet ;  and, 
from' the  observations  of  ten  years,  this  distinguished 
meteorologist  has  been  led  to  believe,  that  it  is  entirely 
composed  of  snow-flakes.     Indeed,  the  temperature  of 
the  elevated  regions  in  which  it  floats,  must  be  often  far 
DC  low  the  freezing  point. 

238.  CUMULUS.      This   kind   of  cloud   acquires   its 
name  from  the  Latin  word  cumulus  or  heap  ;  the  vapor 
seeming  to  be  piled  or  heaped  together.     It  is  usually- 
seen  in  the  form  of  a  hemisphere,  resting  upon  a  hori- 
zontal base  ;  but  at  times  detached  masses  gather  into 
one  vast  cloud  upon  the  horizon  ;  their  radiant  sum- 
mits gleaming  like  the  snowy  peaks  of  distant  moun- 
tains, (figure  13.  6.) 

239.  The  cumulus  \s  \\\e  cloud  of  day :  and  is  produced 
by  the  ascending  currents  of  warm  air.  caused  by  the 
solar  heat.    During  the  fine  days  of  summer,  its  peculiar 
figure  is  most  perfect,  and  its  formation  and  decline  oc- 
cur in  the  following  manner.     Although  the  sun  may 
have  arisen  in  a  cloudless  sky,  a  few  solitarv  specks  of 
vapor  may  be  seen  towards  eight  or  nine  o'clock  ;  these, 
as  the  day  advances,  enlarge  from  within,  become  thicker, 
and  accumulate,  in  rounded  masses,  which  continue  to 
increase  in  number  and  si-?-,,  till  the  hottest  part  of  the 
day.     After  th-s  time  they  gradually  lessen,  and  often 
entirely  vani?l«,  /caving  the  sky  at<sunset  again  perfectly 
berene. 

210.  The  civmilus  floats  low  in  th.<}  iron: ing;  but  it3 

How  may  its  peculiar  structure  be  best  discerned  } 

How  far  above  the  general  surface  of  the  earth  does  ;ho  c.'rris  riie  t 

Of  what  does  it  consist  according  to  Kaemtz  1 

Describe  the  cumulus. 

How  dors  it  original.;? 

Describe  the  modt  of  its  formation  and  the  changes  it  undergoes  1 


CLASSIFICATION. 


101 


altitude  increases  with  that  of  the  ascending  currents, 
which  attain  their  highest  elevation  soon  after  mid-day  ; 
towards  evening  the  current?  subside,  and  the  cloud 
descends.  This  circumstance  has  often  been  remarked 
by  meteorologists,  when  stationed  on  elevated  moun 
tains.  In  the  morning,  the  cumulus  has  been  seen  be- 
neath them;  it  enveloped  them  towards  noon;  then 
soared  above  them  for  several  hours,  and  descended  to 
the  vale  at  the  close  of  day. 

Fig.  13. 


CIRRUS  (fl),  CUMULUS  (6),  AND  STRATUS  (C). 


What  is  said  of  its  height  in  the  morning,  at  mid-day,  and  in  the  even- 
ing? 

Account  for  the  facts  stated  in  IT  24Q, 


102  AQUEOUS    PHENOMENA. 

211.  It.  is  not  difficult  to  account  for  the  facts  just  de- 
tailed. The  cumulus  begins  to  be  formed,  when  the 
warm  currents,  in  their  upward  progress,  arrive  at  a 
temperature  so  low  that  tli«y  become  over-saturated 
with  moisture ;  and  the  excess  is  then  condensed  into  a 
cloud. 

The  higher  the  currents  rise,  the  colder  is  the  atmos- 
phere, generally  speaking,  and  the  cloud  must  necessa- 
rily enlarge  ;  but  when  in  the  afternoon  the  strength 
of  the  currents  abates,  the  clouds  which  are  buoyed  up 
by  their  force,  sink  down  into  warmer  regions  of  the  at- 
mosphere, and  are  either  partially  or  completely  dis- 
solved. 

242.  The  rounded  figure  of  the  cumulus  is  attributed 
by  Saussure  to   the  mode  of  its  formation  ;    for  when 
one  fluid  flows  through   another  at  rest,  the  outline  of 
the  figure  assumed  by  the  first  will  be  composed  of 
curved  lines.     This  may  be  seen,  by  suffering  a  drop  of 
milk,  or  ink,  to  fall  into  a  glass  of  water ;  but  the  shape 
of  a  cloud  of  steam,  as  it  issues  from  the  boiler  of  a 
locomotive,  presents  a  far  better  illustration. 

243.  STRATUS.     This  cloud  derives  its  name  from 
the  Latin  word  stratus,  or  covering  ;   it  forms  about 
sunset,  increases  in  density  during  the  night,  and  dis- 
appears at  sunrise.     It  is  caused  by  the  vapors  which 
have  been  exhaled  during  the  heat  of  the  day,  but  re- 
turn again  to  the  earth  towards  the  evening,  when  the 
temperature  has  declined,  and  are  then  condensed  into 
a  sheet  of  clouds,  which  stretch  along  and  rest  upon  the 
horizon  (figure   13.,  c).      This  class  likewise   includes 
those  light  and  spreading  mists,  which  gather  in  mead- 
ows and  vales  in  the  evening  of  a  warm  summer's  day, 
floating  like  a  veil  over  the  surface  of  the  ground,  and 
extending  but  a  short  distance  above  it. 

244.  CIRRO-STRATUS.     This  cloud  is  so  called,  be 


What  causes  the  rounded  figure  of  the  cumulus! 
Describe  the  stratus. 

When  does  it  form,  increase  and  vanish  ? 
How  does  it  originate  7 


CLASSIFICATION.  103 

cause  it  partakes  of  the  characteristics  of  the  cirrtu  and 
stratus  ;  originating  usually  in  the  cirrus.  It  is  remark- 
able for  its  great  length,  in  proportion  to  its  thickness ; 
but  though  preserving  in  the  main  this  peculiarity,  it 
assumes  many  varieties  of  form. 

245.  At  one  time  it  consists  of  a  number  of  parallel 
bars  of  vapor,  in  close  proximity,  blended  together  at 
the  middle,  but  separated  at  the  edges  (figure  14.,  6),  or 
it  may  appear  as  a  streak  of  vapor,  broadest  at  the  mid- 
dle, and  tapering  towards  either  end  (c).     A  third  va- 
riety consists  of  small  rows  of  clouds,  parallel  to  one 
another ;    each  successive  row  becoming  shorter,  front 
the  widest   part  of  the  cloud  to  the    extremities,  (d.} 
The  name  of  cirro-stratus  is  also  given   to  that  thin, 
gauze-like    cloud,    which    sometimes   overspreads    the 
whole  sky,  and  through  which   the  sun  and  moon  are 
dimly  visible. 

246.  CIRRO-CUMULUS.     It  not  unfrequently  happens, 
that  the  heavens  appear  as  if  sown  with  little  round 
masses  of  clouds,  lying  near  to  each  other,  but  perfectly 
separated  by  intervals  of  sky  (figure  14.,  a).     This  cloud 
is  the  cirro-cumulus,  and  often  arises  from  a  change  in. 
the  cirrus  and  cirro-stratus  ;  the  bars  of  the  latter  being 
divided  across  the  direction  of  their  length,  and  the  dif- 
ferent parts  rounding  into  the  cirro-cumulus.      Some- 
times the  reverse  occurs,  and  the  cirro-cumulus  is  seen 
changing  into  the  cirrus  and  cirro-stratus. 

247.  The  structure  of  the  cirro-cumulus  is  not  always 
the  same :  at  one  time  the  masses  are  very  dense  and 
well-rounded ;  at  another  their  form  is   irregular,  and 
the  sky  often  presents  a  curdled  appearance,  when  cov- 
ered with  this  cloud.     Sometimes  the  cirro-cumulus  is 
so  light  and  fleecy,  that  the  rays  of  the  sun,  as  they  tra- 
verse, it,  are  scarcely  dim.med.     Humboldt  found  them 

Describe  the  cirro-stratus. 

How  is  it  produced  1 

What  are  some  of  its  varieties  1 

Describe  the  cirro-cumulus. 

Whence  does  it  arise  ? 

State  some  of  the  peculiarities  of  this  cloud 


104 


AQUEOUS    PHENOMENA. 


even  so  delicate  that  he  was  able  to  discern  through 
them  the  spots  on  the  moon.  The  last  two  classes  of 
clouds,  like  the  cirrus,  float  at  a  very  lofty  height. 


Fig.  14. 


CIRKO-STRATDS  (6,  C,  </),  CIRRO-CUMULUS  (a),  NIMBUS  (C),  CUMULO-STRATUS  (/). 


2  IS.  CUMULOHBTRATUS,      The   variety  of  cloud  to 
which  this  name  is  given,  combines  the  characteristics 
of  the  cumulus  and  stratus.    -Its  base  consists  of  a  hor- 
izontal stratum  or  layer  of  vapor,  from  which  rise  large. 
overhanging  masses  of  cumulus  (figure  14../).     Some- 
what is  said  respecting  the  height  of  the  cirro-stratus  and  euro-cumulus  1 
Describe  the  cumulo-stratus. 
Of  what  docs  it  consist  ? 


CLASSIFICATION.  105 

times  contiguous  cumulus  clouds  unite,  and  passing  into 
the  state  of  cumulo-stratus,  form  groups  of  immense 
size.  This  cloud  is  seen  in  perfection  upon  the  ap- 
proach of  a  thunder-storm,  when  the  cumulus  clouds, 
driven  together  by  the  wind,  are  piled  upon  each  other, 
and  assume  those  peculiar  forms  commonly  known  by 
the  name  of  thunderheads. 

249.  This  modification  also  frequently  arises,  when 
the  cumulus  is  pierced  by  the  cirro-stratus ;  and  it  is  by 
no  means  unusual  to  see  four  or  Jive  parallel  bars  of  the 
cirro-stratus,  one  above  the  other,  passing  through  the 
same  pile  of  clouds,  which  then  present  successive  tiers 
of  the  cumulo-stratus. 

250.  NIMBUS,  OR   RAIN-CLOUD.     This  cloud   is   so 
called  from  the  Latin  word,  nimbus,  a  rainy  dark  cloud  ; 
it  possesses  no  peculiarity  of  form,  but  is  distinguished  by 
its  uniform  gray   tint  and  fringed  edges  (figure  14.,  e). 
It  is  usually  composed  of  some  of  the  preceding  classes 
of  clouds,  so  blended  together  that  they  cannot  be  dis- 
tinguished, and  is  produced  by  a  change  in  their  struc- 
ture, the  result  of  an  increase  in  density. 

251.  The  nimbus  often  originates  in  the  cumulo-stra- 
tus, which,  as  it  increases  in  thickness,  frequently  as- 
sumes a  black  or  bluish  tint.     In  a  short  time  this  hue 
changes   to  gray,  a  circumstance  which  indicates  thai 
the  nimbus  is  formed  and  rain  descending. 

When  is  this  cloud  most  perfectly  formed  ? 

Under  what  other  circumstances  is  the  cumulo-stratus  seen  1 

Describe  the  nimbus. 

How  is  it  distinguished  1 

Of  what  does  it  consist  7 

How  is  it  caused  ? 

In  what  cloud  does  it  often  originate  1 

What  does  a  gray  tint  indicate  1 

5* 


106  AClUEOUS    PHENOMENA. 

CHAPTER   IV. 

OF   DEW. 

252.  DEW  is  the  moisture  spontaneously  deposited 
upon  the  surfaces  of  bodies  exposed  to  the  atmosphere, 
when  the  latter  is  free  from  the  presence  of  fogs  ana 
rain. 

253.  The  whole  subject  of  dew  was  most  happily 
illustrated  by  the  observations  and  experiments  of  Dr. 
Wells,  in  1812 ;  and  the  theory  which  he  then  advanced 
is  now  generally  received,  supported  as  it  is  by  a  vasl 
assemblage  of  facts. 

254.  DEPOSITION.     The  deposition  of  dew  is  caused 
by  the  unequal  radiation  of  heat  from  the  atmosphere 
and  the  substance  bedewed.    During  the  day,  the  bodies, 
that  either  compose  the  solid  crust  of  the  earth  or  clothe 
its   surface,  become  heated  by  the  solar  rays,  and  the 
lower  stratum  of  that  portion  of  the  atmosphere  which  is 
directly  above,  is  then   likewise  raised  in  temperature, 
and  its  capacity  for  moisture  increased. 

When,  however,  the  night  comes  on,  and  even  before, 
the  earth  and  air,  radiating  their  acquired  heat  into  free 
space,  sink  in  temperature  ;  but  the  loss  of  the  former  is 
greater  than  that  of  the  latter,  since,  during  the  night,  as 
experiments  show,  the  air  a  few  feet  above  the  ground, 
is  sometimes  warmer  than  the  surface  of  the  soil,  by  fif- 
teen degrees. 

It  thus  occurs,  that  the  stratum  of  air  immediately  in 
contact  with  the  earth  is  cooled  down  by  the  latter,  be- 
yond the  point  of  saturation  ;  and  the  excess  of  humidity 
is  condensed,  upon  the  substances  that  form  its  surface, 
in  drops  of  dew.  (Art.  65.) 

255.  It  may  therefore  be  assumed  as  a  principle,  that 
dew  never  begins  to  be  deposited  upon  the  surface  of 

What  is  the  subject  of  chapter  fourth?    Define  dew. 
Whose  theory  is  generally  received  ? 

How  is  the  deposition  of  dew  caused  ?    Explain  the  process. 
How  much  warmer  is  the  air  sometimes  than  the  ground  1 
What  principle  may  be  assumed  7 


INFLUENCE    OF    THE    ATMOSPHERE.  107 

any  body,  until  it  is  colder  than  the  contiguous  atmos- 
phere ;  and,  other  circumstances  being  the  same,  the 
greater  this  difference  in  temperature,  the  greater  the 
amount  of  dew. 

The  quantity  of  dew  deposited  within  any  given  time, 
depends,  chiefly,  upon  the  humidity ',  serenity,  and  tran- 
quillity of  the  atmosphere  ;  and  the  constitution,  form, 
surface,  and  location  of  the  bodies  receiving  the  moisture. 

INFLUENCE  OF  THE  CONDITION  OF  THE  ATMOSPHERE. 

256.  HUMIDITY.     That  the  quantity  of  latent  vapor 
in  the  air  must  regulate,  in  a  great  measure,  the  amount 
of  dew,  is  perfectly  clear,  since  the  latter  is  nothing  else 
than  condensed  atmospheric  vapor. 

257.  SERENITY.     Every  thing  that  favors  radiation 
from  the  earth,  and  consequently  produces  an  increase 
of  cold,  contributes  to  the  formation  of  dew.     Thus  it 
is  copiously  deposited  on  serene  nights ;  for  the  radia- 
tion from  the  earth  then  proceeds  unchecked  :  while,  on 
the  contrary,   little  or   no  dew  is  seen  after  a  cloudy 
night ;  since  the  canopy  of  the  clouds  reflects  back  to 
the  earth  the  heat  that  is  proceeding  from  it,  maintain- 
ing its  surface  and  the  contiguous  air  at  nearly  the  same 
temperature. 

If,  however,  the  clouds  separate  only  for  a  few  mo- 
ments, the  heat  escapes  from  the  earth  through  the  in- 
tervals, and  dew  is  rapidly  deposited  ;  but  if  the  sky  is 
again  suddenly  overcast,  the  radiation  is  arrested,  and 
the  heat  reflected  back  to  the  earth,  raising  the  tem- 
perature at  its  surface,  and  speedily  evaporating  the  dew 
just  formed. 

258.  These   singular  changes  in   temperature  were, 
observed   by  Dr.  Wells.     On  one  night,  the  sky  being 
clear,  the  temperature  of  the  grass,  at  half  past  nine, 
was  32°  Fah. ;  in  twenty  minutes  afterwards,  the  heav- 
ens being  suddenly  overcast,  it  rose  to  39°  Fah. ;  in 


What  circumstances  influence  the  quantity  of  dew  .* 
What  is  the  effect  of  humidity?  What  of  serenity! 
What  is  the  influence  of  clouds  ?  Give  instances. 


108  AQUEOUS    PHENOMENA 

firf/ify  minutes  more,  under  a  serene  sky.  it  sunk  again 
to  32°  Fah.  It  was  also  found,  that  a  thermometer  ly- 
in«-  upon  the  grass,  would  rise  several  degrees,  if  the  sky 
directly  above  it  was  covered  by  a  cloud  only  for  a  few 
minutes.  The  influence  of  clouds  upon  the  tempera- 
ture of  the  air  is  by  no  means  as  great ;  for  while,  on  one 
evening,  when  the  sky  was  obscured  for  the  space  of 
fortv-five  minutes,  a  thermometer  placed  upon  the  turf 
rose  fifteen  degrees,  another,  suspended  in  the  atmos- 
l>lit're  just  above,  rose  but  three  and  a  half  degrees. 

'  250.  TRANQUILLITY.  In  a  calm  night,  other  circum- 
stances being  the  same,  more  dew  will  be  deposited 
than  when  it  is  windy  ;  for  the  wind  will  not  suffer  any 
one  portion  of  air  to  remain  long  enough  in  contact  with 
the  cold  surface  of  any  body  to  condense  much  of  its 
moisture,  hurrying  it  away  before  it  is  sufficiently  cooled 
down  for  this  purpose. 

260.  A  slight  agitation  of  the  atmosphere,  however, 
is  of  advantage  ;  since,  after  one  portion  of  air  has  de- 
posited upon  any  surfacejts  exuberant  moisture,  it  re- 
moves it  from  the  spot,  bringing  up  a  fresh  portion  to 
the  same  place,  and  so  on  successively ;  giving  time  to 
e;'<-h  to  sink  to  the  temperature  of  the  surface  bedewed. 
.As  the  night  advances,  and  the  earth  becomes  still 
colder,  the  same  volumes  of  air,  renewing  their  contact 
with  the  same  surface,  may  be  again  surcharged  with 
humidity,  and  deposit  more  dew. 

201.  EVENING  AND  MORNING.  Dew  is  often  formed 
towards  the  close  of  the  afternoon,  in  consequence  of 
the  earth  then  losing  more  heat  by  radiation  than  it 
receives  from  the  slanting  rays  of  the  descending  sun. 
It  ;ilso  frequently  forms  in  shady  places  just  after  sun- 
ri*e  ;  for  the  surface  of  the  globe,  which  has  been  grad- 
ually sinking  in  temperature  during  the  night,  is  not 

Which  is  most  affected  by  clouds,  the  air  or  the  ground  1    Illustrate. 
Why  dot's  a  wind  lessen  the  amount  of  dew  1 
What  is  the  effbct  of  a  slight  agitation  of  the  air? 
Why  does  dew  begin  to  form  towards  the  close  of  day  7 
Where  does  it  form  after  sunrise? 


SUBSTANCE    BEDEWED.  109 

immediately  influenced  by  the  warm  beams  of  the  sun. 
Indeed,  at  this  time,  more  dew  is  deposited  than  at  any 
other  equal  period  in  the  twenty-four  hours. 

INFLUENCE  OF  THE  SUBSTANCE  BEDEWED. 

262.  CONSTITUTION.  Since  the  production  of  dew 
requires  that  the  body  bedewed  must  be  colder  than  the 
surrounding  atmosphere,  all  substances,  which  rapidly 
lose  their  own  heat  and  slowly  acquire  that  of  others, 
are  susceptible  of  being  copiously  bedewed.  On  the  con- 
trary, substances  possessing  the  opposite  qualities  con- 
tract but  little  dew. 

Under  the  first  class  may  be  included  glass,  silk, 
down,  wool,  and.  in  general,  all  bodies  of  a  porous  tex- 
ture; while  metals  and  rocks  belong  to  the  second  divi 
sion. 

263..  If  similar  plates  of  polished  glass  and  metal  are 
exposed  alike  upon  the  soil  during  a  favorable  night, 
in  the  morning  the  glass  will  be  drenched  with  dew,  but 
the  brightness  of  the  metal  ^vvill  be  scarcely  dimmed. 
These  different  results  arise  from  the  fact,  that,  while 
the  glass  is  deprived  by  radiation  of  ninety  hundredths 
of  its  original  heat,  twelve  hundredths  is  all  that  the 
metal  loses.  Besides,  the  glass,  being  a  bad  conductor, 
draws  but  little  warmth  from  the  earth  to  supply  its  loss  ; 
while  the  metal,  being  a  good,  conductor,  can  easily 
restore  any  reduction  of  heat  from  the  warm  soil  imme- 
diately below. 

Large  plates  of  metal,  exposed  throughout  the  night, 
never  acquire  a  temperature  more  than  three  or  four 
degrees  below  that  of  the  atmosphere. 

264.  SURFACE  AND  FORM.  A  polished  surface  does 
not  radiate  so  well  as  one  that  is  rough  and  uneven; 
and  the  latter  is  always  found,  under  a  like  exposure, 
to  receive  more  dew.  Points  radiate  heat  most  rapidly, 

Account  for  its  deposition  at  this  time. 

What  substances  are  capable  of  being  copiously  bedewed  7 

What  not  1    Give  examples. 

Account  for  the  unequal  deposition  of  dew  on  glass  and  mefal. 

What  is  said  of  polished  and  rough  surfaces  in  this  particular  1 


HO  AQUEOUS    PHENOMENA. 

and  are  therefore  the  coldest  portions  of  a  radiating 
body  ;  hence,  of  all  the  globules  of  dew  that  form  upon 
blades  of  grass,  the  largest  are  found  at  the  very  ex- 
tremities. 

Grass  is  well  known  to  be  copiously  bedewed  ;  its 
form,  as  just  mentioned,  causes  it  to  lose  its  own  warmth 
with  great  rapidity,  while  its  porous  texture  prevents 
it,  at  the  same  time,  from  replenishing  its  loss  from  the 
soil. 

265.  LOCATION.  If  a  body  is  screened  from  the  open 
sky,  it  contracts  less  dew  than  when  fully  exposed  ;  for 
the  screen  arrests  radiation  in  the  manner  of  clouds  ; 
and  the  difference  in  temperature  between  the  sheltered 
body  and  the  contiguous  air,  is  less  than  that  which 
would  exist  between  the  same  body  and  the  surround- 
ing atmosphere,  were  the  substance  bedewed  entirely 
unsheltered.  This  has  been  proved  by  the  experiments 
of  Dr.  Wells. 

260.  In  four  trials,  made  with  two  parcels  of  wool,  in 
all  respects  alike, the  h'rsf  of  which  was  placed  upon  the 
upper  side  of  a  board,  four  feet  from  the  ground,  and  the 
second  loosely  attached  to  the  under  side,  the  gain,  in 
Hew,  was  as  follows  : 

1st  night.  2d.  3d.  4th. 

grs.  gre.  grs.  %n. 

1st  parcel,  14  19  11  20 

2d    do.  4  6  2  4 

We  hence  perceive,  why,  beneath  the  shelter  of  trees, 
and  on  the  under  surfaces  of  leaves,  but  little  dew  is 
tbund. 

267.  Dew  has  never  been  found  upon  the  surface  of 
large  bodies  of  water  ;  for  whenever  the  aqueous  parti 
cles  at  the  surface  are  cooled,  they  become  heavier  than 
those  below  them,  and  sink  ;  while  warmer  and  lighter 
particles  rise  to  the  top.  These,  in  their  turn,  become 

What  of  points  1 

Why  are  the  largest  beads  of  dew  upon  the  end  of  the  blades  of  grass? 
Why  does  an  exposed  body  contract  more  dew  than  one  which  is  shel- 
tered 1    Give  the  results  of  Dr.  Wells'  experiments. 
Why  are  the  surfaces  of  large  bodies  of  water  free  from  dew  1 


LOCATION    AND    COLOR.  Ill 

heavier  and  descend  ;  and  the  process  continuing  through- 
out the  night,  maintains  the  surface  of  the  water  and 
the  air  at  nearly  the  same  temperature. 

Dr.  Wells  ascertained,  by  experiment,  that  even  a 
small  quantity  of  water  gains  no  weight  by  exposure 
during  a  single  night. 

It  appears,  from  the  narrative  of  the  U.  S.  Exploring 
Expedition,  and  from  other  sources,  that  on  the  ocean 
heavy  deposits  of  dew  sometimes  occur  upon  the  decks 
of  vessels. 

268.  The  exposed  parts  of  the  human  body  are  never 
covered  with  dew  ;  since  the  vital  heat,  varying  from  96° 
to  98°  Fah.,  effectually  prevents  such  a  loss  of  warmth 
as  is  necessary  to  its  production. 

269.  COLOR.     A  few  experiments  were  made  by  Dr. 
Wells,  in  order    to  ascertain   the  effect  of  color  upon 
dew  ;  but  without  any  decisive  results.     In  1833,  Dr, 
Stark,  of  Edinburg,  made  two  experiments,  from  which 
he  inferred,  that  under  like  exposures,  more  dew  was  de- 
posited upon  dark-colored  bodies,  than  upon  Zt^A^-colored. 
But  the  author  of  this  work,  from  an  investigation  pros- 
ecuted by  himself  during  the  summer  of  1846,  arrived 
at  the  conclusion,  that  color  exerts  no  influence  what- 
ever upon  the  quantity  of  dew.    This  fact  might  also  be 
inferred  from   the  experiments  of  Dr.  Bache   on  heat, 
which  clearly  show,  that  the  amount  of  radiation  is  not 
affected  by  color. 

270.  OBSERVATIONS.    The  observations,  which  have 
been  made  in  various  regions  of  the  globe,  in  regard  to 
the  occurrence  of  dew,  strongly  corroborate  the  theory 
of  Dr  Wells.     In  Bengal,  during  the  month  of  Novem- 
ber, the  nights  are  beautifully  serene,  and  accompanied 
with  heavy  dews  ;  but  in  April  and   May,  when  high 
winds  prevail,  with  a  close  and  cloudy  atmosphere,  no 

What  experiment  was  made  by  Dr.  Wells? 

What  is  stated  in  the  narrative  of  the  Exploring  Expedition  ? 

Why  is  dew  never  found  upon  the  human  body  1 

What  is  said  as  to  the  influence  of  color  1 

What  do  the  observations  made  in  different  regions  attest  ? 

Give  instances. 


112  AQUEOUS    PHENOMENA. 

dew  is  deposited.  From  September  to  March,  the  sun 
glows  like  an  orb  of  fire  over  Southern  Guinea  ;  but  the 
ni«h(s  are  coo/,  and  the  parched  soil  is  refreshed  with 
abundant  dews.  In  Egypt,  profuse  dews,  like  rain, 
occur  in  the  summer,  when  the  nights  are  resplendent 
with  stars ;  while  at  Thebes,  where  the  sky  is  con- 
stantly serene,  abundant  dews  are  the  only  moisture 
that  the  earth  receives  from  above,  during  the  lapse  of 
many  years. 

271.  FACTS  EXPLAINED.    The  explanation  of  several 
familiar  facts,  depends  upon   the    foregoing  principles. 
Thus,  for  instance,  if,  in  a  warm  summer's  day,  a  glass 
is  filled  with  cold  water,  the  exterior  surface  is  seen 
covered  with  moisture ;  for  the  reason,  that  the  glass, 
being  colder  than  the  air  in  contact,  cools  the  latter  be- 
low the  dew-point.     In  summer,  caves  and   cellars  are 
damp ;    because  the  warm   air  that  enters  them  from 
without  is  cooled  down,  and  its  humidity  either  floats 
in  the  atmosphere,  or  is  condensed  in  beads  of  dew  upon 
the  stones. 

272.  BENEFICENT   DISTRIBUTION.      The  mode  in 
which  the  blessing  of  dew  is    dispensed  to  the  earth, 
beautifully  exemplifies  the  benevolence  of  our  Creator. 

In  the  temperate  climes,  where  the  frequent  inter- 
change of  sun  and  shower  preserves  the  earth  from  the 
extremes  of  heat  and  moisture,  very  little  dew  is  needed, 
and  but  comparatively  little  is  deposited.  The  regions 
however  within  the  tropics  are  deprived. of  rain  for 
months,  and  this  destitution  is  partially  supplied  by 
the  dews,  which  precisely  at  these  seasons  are  most 
abundant. 

273.  The  lake  and  the  river  are  not  visited  by  de\v, 
for  no  form  of  vegetation  exists  within  them  that  needs 
its  presence.     To  the  naked  rock  it  comes  but  in  scanty 
measure ;  for  there  is  nothing  here  to  nourish — save, 
perhaps,  the  thorny  cactus,  which,  from  its  very  form  and 


What  facts  are  explained  upon  the  foregoing  principles  1 
What  does  the  distribution  of  dew  exemplify  1 
Give  the  various  illustrations. 


DISTRIBUTION.  113 

nature,  is  adapted  to  its  situation  ;  and  though  spring- 
ing from  the  cleft  of  a  rock  beneath  a  tropic  sun,  or 
striking  its  roots  in  the  sands  of  the  desert,  is  capable 
of  deriving  from  the  air  an  adequate  supply  of  moisture. 

274.  Upon  the  foliage  of  the  grove  very   little  dew  is 
deposited,  in  consequence  of  the  inclined  position  of  the 
leaves,    their   frequent    motion,   and    the    shelter    they 
a  fiord  each  other.     Nor  is  it  needed  ;  for  the  natural 
supply  of  moisture  rises  deep  from  the  soil  through   the 
parent  trunk,  diffusing  itself  throughout  every  branch 
to  the  remotest  extremity  of  the  slenderest  bough. 

275.  The  dew,  however,  blesses,  in  all  its  invigorating 
exuberance,   the  humble  plant  and  lender  herbage,  a 
vast  class  of  vegetable  life,  at  once  the  most  perishable 
and  the  most  useful ;  it  is  the  first  of  all  to  feel  the  effects 
of  drought,  and  yet  it  is  that  which  is  necessary  to  the 
very  existence  of  man.     From  the  jield,  not  from  the 
forest,  comes  our  support ;  and   the  failure  of  a  single 
plant,  the  grass  or  the  bladed  grain,  may  bring  upon  a 
nation  scarcity  and  famine. 


CHAPTER   V. 

OF   HOAR-FROST   AND    SNOW. 

276.  HOAR-FROST.  Hoar-frost  is  produced  in  the 
same  manner  as  dew.  Late  in  the  spring,  and  early  in 
the  fall,  the  surface  of  the  earth,  during  serene  nights, 
sinks  in  tempe;  attire  below  the  freezing-  point,  while  the 
atmosphere,  a  few  feet  above,  is  higher  by  several 
degrees. 

The  moisture  which  is  then  deposited  becomes  con- 
gealed in  sparkling  crystals ;  and  the  stems  of  plants 
and  the  branches  of  low  shrubs  are  often  adorned  with 
fringes,  formed  of  the  most  beautiful  and  delicate  crys- 
tallizations. 


What  is  the  subject  of  chapter  fifth  1 
How  is  hoar-frost  produced  1 
Describe  its  appearance. 


114  AQ.UEOUS    PHENOMENA. 

277.  A  species    of  hoar-frost    occurs  when  a  warm 
south  wind    succeeds  a   continuance  of  cold  weather. 
Stone  columns   and  buildings   are   then    covered  with 
a  snowy   incrustation,  composed  of  an  assemblage  of 
minute  crystals,  caused    by  the   influence  of  the  low 
temperature  of  the  stone  upon  the  condensed  vapor  of 
the  air. 

The  effect  of  a  cold'body  upon  moist  air-is  well  shown 
by  the  following  facts  related  by  Ballantyne,  who  resided 
for  two  years  at  York  Factory,  in  the  vicinity  of  Hud- 
son's Bay.  After  narrating  the  adventures  of  a  hunting 
expedition  in  the  depth  of  winter,  he  thus  describes  an 
incident  that  occurred  upon  the  return  of  himself  and 
his  companions  to  their  dwelling.  "It  was  curious  to 
observe  the  change  that  took  place  in  the  appearance  of 
our  guns  after  we  entered  the  warm  room.  The  barrels 
and  every  bit  of  metal  upon  them  instantly  became 
white,  like  ground  glass.  This  phenomenon  was  caused 
by  the  condensation  and  freezing  of  the  moist  atmos- 
phere of  the  room  upon  the  cold  iron.  Any  piece  of 
metal,  when  brought  suddenly  out  of  such  intense  cold 
into  a  warm  room,  will  in  this  way  become  covered  with 
a  pure  white  coating  of  hoar-frost.  It  does  not  remain 
long  in  this  state,  however,  as  the  warmth  of  the  room 
soon  heats  the  metal  and  melts  the  ice.  Thus,  in  about 
ten  minutes  our  guns  assumed  three  different  appear- 
ances. When  we  entered  the  house  they 'were  clean, 
polished,  and  dry  ;  in  rive  minutes  they  weie  as  white  as 
snow,  and  in  five  more  were  dripping  \\ef" 

278.  Every  thing  that  prevents  the  rcu  nation  of  heat, 
fl/Tc.\s7s  the  formation  of  hoar-frost.     During  the  chilly 
nights  of  spring,  plants  that  are  sheltered  by  trees  are 
less  liable  to  be  frozen  than  those  which   are  fully  ex- 
posed ;  and  a  slight  covering  of  straw,  or  even  of  paper, 
will  often   afford   an   effectual   protection.      Vineyards 
have  frequently  been  saved  from  the  effects   of  frost,  by 
enveloping  them  during  the  night  in  a  cloud  of  smoke. 

Wliit  effect  is  caused  by  a  warm  south  wind,  after  a  period  of  cold  wea- 
ther 1     Relate  the  facts  related  by  Ballantyne. 
What  arrests  the  formation  of  hoar-frost  1 


HOAR-FROST    AND    SNOW.  116 

279.  The  effect  of  a   screen   in  checking  radiation, 
and  thus  preventing  frost,  has  been  finely  illustrated  by 
the  experiments  of  David  Scott,  of  India.     Throughout 
the  whole  region  of  Upper  India,  ice  is  artificially  pro- 
cured   by  placing    upon   a  layer  of  dry  straw,  in  the 
bottom  of  small  pits,  and  fully  exposed  to  the  clear  sky, 
broad,  shallow  earthen  pans,   filled  with  water.     Such 
is  the  radiation  during  the  night,  that  a  thin  crust  of  ice 
will  sometimes  form  upon  the  water,  when  the  tempera- 
ture of  the  air  on  a  level  with  the  pits  is  as  high  as 
41°  Fah. 

On  one  occasion,  Mr.  Scott  extended  a  muslin  turban 
across  a  pit,  three  feet  above  the  pans.  No  ice  was 
formed  in  the  vessels  immediately  under  it ;  but,  in  sev- 
eral that  were  partially  covered,  ice  appeared  upon  the 
part  of  the  water  beyond  the  shelter  of  the  muslin ; 
while  the  surface  beneatli  the  turban  remained  in  a  fluid 
state.  Two  strings,  crossing  each  other  at  a  lower 
height  above  a  pan,  under  favorable  circumstances, 
divided  the  ice  into  four  quarters,  the  water  beneath  the 
strings  continuing  unfrozen. 

SNOW. 

280.  Snow  is  the  frozen  moisture  that  descends  from 
the  atmosphere  when  the  temperature  of  the  air  at  th.e 
surface  of  the  earth  is  near  or  below  the  freezing  point. 

281.  SNOW-FLAKE.     At  moderate  heights,  and  in  the 
temperate  regions,  snow  commonly  falls    after  several 
days  of  severe  frost  when   the  weather  has  moderated. 
The  largest  flakes  occur  when  the  air   abounds  with 
vapor  and  the  temperature  is  about  32°  Fah.  ;  but  as 
the    moisture  diminishes,  and   the  cold    increases,  the 
snow  becomes  finer. 

In  the  former  case,  it  is  not  unusual  to  observe 
flakes  an  inch  in  diameter  ;  and  in  the  latter,  they  only 
measure  a  few  hundredths  of  an  inch. 

Illustrate  the  influence  of  a  screen,  by  the  experiments  of  Scott. 

Define  snow.     When  does  it  usually  fall  1 

Under  what  circumstances  do  the  largest  flakes  occur  1 

Under  what  circumstances  do  the  smallest  ?    How  large  are  they  ? 


116  AQUEOUS    PHENOMENA. 

At  Bossekop  a  fall  of  snow  occurred  when  the  thermo- 
meter stood  at  10°  Fab.,  and  the  diameter  ol  the  flakes 
then  scarcely  exceeded  seven  hundredths  of  an  inch. 

The  snow-flake  is  composed  of  regular  crystals,  and 
its  beautiful  figures  and  rich  diversity  of  forms  have 
ever  excited  the  admiration  of  observers.  In  solid  ice, 
the  crystals  are  so  blended  together  that  theiv  symmetry 
is  lost  in  the  compact  mass  ;  but  in  snow,  they  are  per- 
fectly developed,  when  the  flakes  descend  through  > 
calm  atmosphere.  Any  agitation  of  the  air,  or  an  in- 
crease of  moisture  or  temperature,  destroys  their  deli 
cate  structure. 

If  the  crystals  of  snow  were  solid,  they  would  be 
transparent,  like  other  crystallized  bodies;  but  they 
contain  air.  and  to  this  circumstance  is  attributed  their 
brilliant  whiteness  ;  for  the  air  preventing  the  ready 
transmission  of  light  through  the  snow-flake,  the  rays 
are  copiously  reflected  from  the  assemblage  of  crystals 

The  bulk  of  snow  which  has  just  fallen  is  ten  or 
twelve  times  greater  than  that  of  the  water  obtained  by 
melting  it. 

282.  Though  single  crystals  always  unite  at  angles 
of  30°,  60°.  or  120°,  they  nevertheless  form,  by  their 
different  modes  of  union,  several  hundred  distinct 
varieties. 

Scoresby,  a  celebrated  Arctic  navigator,  has  enu- 
merated six  hundred,  and  delineated  ninety-six;  and 
Kaemtz  has  observed  twenty  more,  not  figured  by 
Scoresby. 

<  283.  SNOW-CRYSTALS.  Although  the  varieties  are 
so  numerous,  they  are  all  comprised  under  five  principal 
classes. 

1st.  Crystals  in  the  form  of  thin  plates ;  they  are 
generally  very  thin,  transparent,  and  of  a  delicate 

How  small  are  they  1 

Of  what  is  the  snow-flake  composed1? 

How  is  the  whiteness  of  snow  caused  1 

What  is  said  of  the  bulk  of  snow? 

State  the  number  of  varieties  of  snow-crystals. 

In  how  many  classes  are  they  comprised  1    Describe  them. 


SNOW-CRYSTALS. 


117 


structure.  This  class  includes  many  remarkable  vari- 
eties, which  are  represented  by  the  first  twentV-five 
figures  of  the  annexed  cuts,  (15.,  16.) 

2d.  Flakes  either  possessing  a  spherical  nucleus,  or 
a  plane  figure,  studded  with  needle-shaped  crystals, 
(figure  26.) 

3d.  Slender  prismatic  crystals ;  usually  six-sided, 
but  sometimes  having  onlv  three  sides. 

4th.  Pyramids  with  six  sides  ;  (figure  27.) 

5th.  Prismatic  crystals,  having,  perpendicular  to  their 
length,  both  at  the  extremities  and  in  the  middle,  thin, 
six-sided  plates ;  (figures  28.,  29.  and  30.)  The  last 


Fig.  15. 


SNOW-CRYSTALS. 


118 


AQUEOUS    PHENOMENA. 

Fiff.  16. 


SNOW-CRYSTALS. 


two  classes  are  extremely  rare,  Scoresby  having  ob- 
served the  fniirth  but  once,  and  the  fifth  only  twice,  in 
all  hi?  voyages. 

Flakes  belonging  to  two  consecutive  falls  of  snow, 
jn  i—ess  different  figures  ;  but  those  which  descend 
during  the  same  storm,  are  usually  alike  in  this  par- 
ticular. 

•'M.  NATURAL  SNOW-BALLS.  Balls  of  snow  are 
sometimes  formed  by  the  action  of  a  high  wind  upon 
light  snow.  Prof.  Cleaveland,  of  Brunswick,  in  Maine, 

What  is  said  of  the  crystals  that  fall  during  the  same  storm  1 


RED    SNOW.  119 

ooserved,  on  the  first  of  April,  1815,  a  great  number  of 
snow-balls  scattered  over  the  fields,  varying  from  one 
to  fifteen  inches  in  diameter.  They  had  evidently  been 
caused  by  the  wind  rolling  up  the  snow,  as  the  track  of 
the  balls  was  distinctly  visible.  In  1830,  similar  balls 
were  seen  by  Mr.  Sheriff,  in  East  Lothian,  scattered 
over  a  wide  extent ;  some  of  the  masses  being  eighteen 
inches  in  diameter. 

285.  But  the  most  remarkable  exhibition  of  this  kind 
was  beheld  by  Mr.  Clarke,  of  Morris  county,  New  Jer- 
sey, in  January,  1808.     A  crust  having  formed  upon 
the  snow  that  had  previously  fallen,  a  light  snow  soon 
after  occurred,  covering  the  glassy  surface  to  the  depth 
of  three-quarters  of  an  inch  ;  the  sky  then  suddenly  be- 
came serene,  and  a  high  wind  arose.    Beneath  the  force 
of  the  gale,  small  portions  of  snow  would  slide  along  for 
the  distance  of  ten  or  twelve  inches,  when  they  would 
begin  to  revolve,  rapidly  increasing  both  in  length  and 
diameter.     Where  the  descent  of  the  ground  favored 
their  formation,  masses  rolled  up  to  the  size  of  a  barrel, 
and,  as  far  as  the  eye  could  see,  the  dazzling  surface 
was  covered  with  balls  and  cylinders  of  snow;  varying 
in  magnitude  from  ten  inches  to  three  feet  in  diameter. 
Upon  examination  they  were  found  to  be  hollow  at  each 
end,  almost  to  the  centre,  and  as  round  as  if  they  had 
been  so  many  logs  of  wood    turned  in  a  lathe.     The 
cylinders  covered  nearly  400  acres,  and  their  number 
was  judged  to  be  nearly  40,000. 

286.  RED  Sxow.     In  1819,  Capt.  Ross  beheld  snow 
of  a  brilliant  crimson  hue,  clothing  the  sides  of  the 
mountains  at  Baffin's  Bay ;  rising,  according  to  his  re- 
port, to  the  heightof  several  hundred  feet,  and  extending 
to  the  distance  of  eight  miles. 

Snow  of  this  tint  is  not,  however,  confined  to  the 
Arctic  regions.  Raymond  had  previously  observed  it  in 
the  Pyrenees.  In  1818,  vast  masses  were  spread  ovei 
the  Italian  Alps  and  Apennines,  and  five  years  before. 


Relate  the  several  accounts  of  natural  snow  balls. 
What  is  said  of  red  snow  1 


120  AQUEOUS    PHENOMENA. 

the  whole  range  of  the  last-mentioned  chain  was  covered 
with  rose-colored  snow.  The  same  phenomenon  was 
seen  by  Scoreshy,  Parry  and  Franklin,  in  high  northern 
latitudes,  and  the  navigators  of  the  southern  hemisphere 
have  found  red  snow  in  great  quantities  at  New  Shet- 
land, 62°  S.  Lat. 

287.  In  snows  of  great  depth,  the  accounts  differ  in 
regard  to  the  thickness  of  the  colored  stratum.     Ross 
conjectured,  that,  in  the  Arctic  mountains,  the  crimson 
hue  penetrated  to  the  depth  of  several  feet  below  the 
surface ;  while  others  could  not  detect  its  existence  be- 
yond one  or  two  inches. 

Among  the  Alps,  the  red  snow  is  usually  discovered 
in  little  sheltered  hollows,  in  layers  not  exceeding  two 
or  three  inches  in  thickness :  though  these  are  some- 
times situated  far  beneath  the  general  surface  of  the 
snow. 

288.  GREEN    SNOW.     When  the  French  meteorolo- 

fists,  Martin  and  Bravais,  traversed  a  field  of  snow  at 
pitzbergen,  in  1838,  it  appeared  of  a  green  hue,  wher- 
ever it  was  pressed  by  the  foot.     The  coloring  matter 
seemed  to  reside  just  below  the  surface,  which  was  bril- 
liantly white. 

Upon  another  excursion,  the  first  observer  beheld  the 
green  particles  spread  like  dust  over  the  snow,  which 
was  also  tinted  green  beneath  the  surface,  and  upon  the 
sides  of  the  field. 

289.  CAUSE.     These  singular  hues  are  produced  by 
the  presence  of  an  infinite  number  of  a  certain  class  of 
microscopic  plants,  which  from  their  great  tenacity  of 
life,   are   capable,  not  only  of  existing  at  a  very  low 
temperature,  but  even  of  nourishing  with  extraordinary 
vigor. 

These  minute  vegetable  forms  are  composed  of  glob- 
ule.s,  which  vary  in  diameter  from  one-thousandth  of  an 
inch  to  one  three-thousandth.  Each  globule  is  divided 
into  seven  or  eight  cells,  filled  with  a  liquid,  in  which 

What  .,!  frrem  anowl 

To  what  cause  are  those  colors  attributed  1 


USES    OF    SNOW.  121 

live  a  great  n  imber  of  animalcules.  The  cells  are  gen- 
erally red,  which  is  supposed  to  be  their  original  color, 
the  green  tint  being  probably  acquired  by  exposure  to 
the  air  and  light. 

These  extraordinary  hues  may,  therefore,  be  regarded 
as  originating  in  the  same  plant,  in  different  stages  of 
development. 

290.  USES  OF  SNOW.  Snow  subserves  many  impor- 
tant purposes.  Gathered  in  exhaustless  stores  upon  the 
high  mountains  of  the  globe,  it  feeds,  as  it  gradually 
melts  beneath  the  heat  of  summer,  thousands  of  rivers, 
which,  flowing  on  from  clime  to  clime,  enrich  the  soil 
and  crown  the  land  with  plenty. 

The  snow-capped  mountains  are  the  natural  refriger- 
ators of  the  glowing  regions  that  lie  within  the  tropics  ; 
cooling  the  winds  that  pass  over  them,  and  mitigating 
the  fierce  temperature  of  the  atmosphere. 

In  the  higher  latitudes,  where  the  winters  are  severe, 
the  snow  forms  a  warm  covering  for  the  soil,  and  de- 
fends vegetation  from  the  rigors  of  the  frost.  It  is  well 
known,  that  grain,  during  an  open  winter,  is  frequently 
destroyed  by  the  cold  ;  and,  in  the  mild  climate  of  Eng- 
land, Alpine  plants  have  perished,  in  consequence  of 
being  deprived  of  their  natural  covering  of  snow. 

During  the  long  night  of  the  polar  climes,  the  inten- 
sity of  the  darkness  is  diminished  by  the  presence  of  the 
snow  ;  inasmuch  as  it  reflects,  instead  of  absorbing,  like 
the  bare  ground,  the  faint,  light  that  there  proceeds  from 
the  sky.  Nor  is  it  to  be  forgotten,  that,  in  these  incle- 
ment regions,  the  wretched  natives  would  be  unsheltered 
during  the  winter,  were  it  not  for  the  snow  ;  since  this, 
when  cut  into  blocks,  supplies  the  Esquimaux  with  the 
means  of  constructing  their  huts. 

\Vhat  is  said  in  regard  to  the  uses  of  srune  1 
6 


122  AQUEOUS    PHENOMENA. 


CHAPTER  VI. 

OP  HAIL. 

291.  HAIL.     The  ice  that  descends  in  showers,  and 
usually  in  summer,  is  called  hail.     It  is  different  from 
sleet,  which  is  nothing  more  than  frozen  rain,  and  oc- 
curs only  in  cold  weather. 

292.  STRUCTURE.     Hailstones   are   generally  pear- 
shaped,  and  if  they  are  divided  through  the  centre,  they 
are  found  to  be  composed  of  alternate  layers  of  ice  and 
snow,  around  a  white,  snowy  nucleus,  resembling  the 
coats  of  an  onion.     The  surface  is  rough,  and  is  some- 
times studded  with  icicles. 

293.  SIZE.     Hail  varies  in  size,  from  stones  as  small 
as  a  pea  to  those  which  are  several  inches  in  circumfer- 
ence.    Benvenuto  Cellini  relates  in  his  memoirs,  that 
during  his  journey  from  Italy  to  France,  he  was  over- 
taken by  a  terrible  storm  in  the  vicinity  of  Lyons  ;  hail- 
stones falling  of  the  size  of  lemons,  and  with  sufficient 
force  to  kill  even  men  and  cattle. 

At  Roncesvalles,  in  August,  1813,  there  fell  upon  a 
division  of  the  British  army  a  storm  of  hail,  in  which 
the  stones  ranged  in  size  from  a  bean  to  a  hen's  egg. 
The  tin  camp-kettles  of  the  soldiers  were  indented  by 
the  masses  of  ice,  some  of  which  were  round,  and  armed 
with  icicles  three  inches  in  length. 

In  May,  1847,  hailstones  of  immense  size  descended 
near  the  town  of  McDonough,  in  Georgia ;  one  of  them 
was  measured  an  hour  after  it  fell  and  found  to  be  ten 
inches  in  circumference.  During  a  terrific  storm,  that 
occurred  at  Cazorta,  in  Spain,  on  the  13th  of  June, 
1829,  the  roofs  of  the  houses  were  broken  in  by  the  hail. 
Some  of  the  stones  are  stated  to  have  weighed  nearly 
four  pounds  and  a  half.  It  is  probable  that  such  extra- 
Define  hail. 

What  is  the  form  and  structure  of  the  hailstone  ? 
What  is  said  of  its  size  1 
Narrate  the  facts  stated. 


HAIL.  123 

ordinary  masses  as  those  which  have  been  mentioned, 
are  formed  by  the  union  of  several  hailstones  frozen  to- 
gether. 

GEOGRAPHICAL   DISTRIBUTION. 

294.  Hailstorms  are  most  frequent  in  the  temperate 
climes,  and  rarely  occur  within  the  tropics,  except  in 
the  vicinity  of  mountains  whose  summits  tower  above 
the  limit  of  perpetual  frost.     Although   by  no  means 
common,  they  are  not  unknown  in  the  high  northern 
latitudes.     Simpson,  on  the  12th  of  August,  1839,  was 
exposed  in  the  straits  of  Boothia,  in  68°  32'  N.  Lat.,  to 
a  tremendous  thunder-storm,  accompanied  with  torrents 
of  rain  and  heavy  showers  of  hail. 

It  is  mostly  in  summer,  and  usually  at  the  hottest  part 
of  the  day,  that  hail  is  observed  to  fall.  Scarcely  any 
occurs  in  the  night. 

ORIGIN. 

295.  The  structure  of  the  hailstone  shows  that  it  is 
not  formed  at  once;  for  the  concentric  layers  around 
the  snowy  nucleus,  consist  of   different  accessions  of 
moisture,  successively  condensed  and  congealed  upon 
the  surface  of  the  stone. 

The  light,  porous  texture  of  the  snowy  centre,  seems 
to  indicate,  that  the  place  of  origin  must  be  some  region 
in  the  atmosphere  where  the  air  is  rare,  and  the  cold  in- 
tense ;  since  the  appearance  of  the  centre  is  similar  to 
that  presented  by  a  drop  of  water,  when  frozen  under 
the  exhausted  receiver  of  an  air-pump. 

296.  It  is  necessary  then  for  the  production  of  hail, 
that  a  warm,  humid  body  of  air  should  mingle  with 
another  so  extremely  cold,  that  their  temperature,  after 
uniting,  shall  be  below  the  freezing  point.     This  com- 
bination must  also  take  place  during  the  warmest  period 

Where  do  hailstorms  fre^aently  occur  1 

Where  rarely  1 

When  do  they  usually  prevail  ? 

What  indicates  that  the  hailstone  is  not  formed  at  once  ? 

Where  must  it  originate  ? 

What  conditions  are  necessary  for  the  production  of  hail? 


124  AQUEOUS    PHENOMENA. 

of  the  year  and  the  day.  In  accounting  for  an  intense 
degree  of  cold  under  such  circumstances,  consists  the 
great*  difficulty  of  explaining  the  phenomena  of  hail- 
storms. 

297.  Until  within  a  few  years,  almost  every  meteor- 
ologist attributed  the  cold  of  hailstorms  to  the  agency 
of  electricity.     It  is  well  known  that  air,  when  electri- 
fied, is  expanded,  and  that  expansion  pioduces  cold.     It 
was  therefore  imagined,  that  the  electrified  state  of  the 
atmosphere  before  a  storm,  caused  such  a  reduction  of 
temperature  as  to  freeze  the  falling  moisture  and  pro- 
duce hail. 

Volta,  a  distinguished  philosopher  of  France,  sup- 
posed the  cold  to  be  the  result  of  evaporation,  but  em- 
ployed electricity  in  a  singular  manner,  as  explained 
below. 

298.  VOLTA'S   THEORY.     According  to  this  theory, 
two  clouds,  differently  electrified,  are  supposed  to  extend 
through  the  sky,  one  directly  above  the  other.    The  cold, 
caused  by  evaporation  from  the  upper  surface  of  the 
lower  cloud,  is  considered  to  be  so  intense,  that  the  vapor 
is  frozen,  and  the  nucleus  of  the  hailstone  then  formed. 
Its  size  is  afterwards  increased  by  the  humidity  it  gathers 
in  vibrating  backwards  and  forwards  between  the  two 
clouds,  like  the  dancing  figures  upon  electrical  plates. 
(C.  969.)    At  last  it  becomes  so  large,  as  to  break  through 
the  lower  cloud,  and  fall  to  the  earth. 

299.  The  sanction  of  a  great  name  gave  weight  to 
this  fanciful  view,  and  in  1821,  throughout  the  southern 
districts  of-  France,  which  are  peculiarly  liable  to  hail 
storms,   hail-rods   were  erected,  in    order   to  draw  the 
electricity  from  the  clouds,  and  thus  protect  the  vine- 
yards.   Their  efficacy,  however,  is  exceedingly  question- 
able. 

The  electric  hypothesis  is,  moreover,  at  variance  with 
facts.  The  forests,  which  constitute  a  vast  assemblage 
of  hail-rods,  are  often  ravaged  by  hail ;  and  in  the  tor- 


What  effect  has  been  attributed  to  electricity  1 
Explain  Volta's  theory. 


ORIGIN.  125 

rid  zone,  where  the  development  of  atmospherical  elec- 
tricity is  greatest,  hailstorms  are  almost  unknown. 

300.  OLMSTED'S   THEORY.     Prof.  Olmsted,  of  Yale 
College,  considers  electricity  as  an  effect,  and  not  the 
cause  of  hailstorms.     According  to  his  theory,  which 
has  been  very  extensively  received,  the  cold' body  of  air 
derives  its  low  temperature,  not  from  electricity,  but  from 
some  known  source  of  cold  ;  and  the  combination,  which 
occasions  the  hail,  may  arise  in  various  ways,  the  prin- 
cipal of  which  appear  to  be  the  following. 

301.  First.     An  exceedingly  cold  wind,  coming  from 
a  region  far  above  the  limit  of  perpetual  frost,  may 
meet  with  a  current  of  warm  air,  blowing  from  a  point 
many  thousand  feet  below  this  limit. 

Secondly.  By  the  force  of  whirlwinds,  large  volumes 
of  warm  air  from  the  surface  of  the  earth  may  be  sud- 
denly transported  into  the  higher  and  colder  regions  of 
the  atmosphere. 

Thirdly.  In  the  vicinity  of  lofty  mountains,  cold 
blasts  are  frequently  known  to  sweep  down  their  sides 
from  the  snowy  peaks  and  glaciers,  and  mingle  with 
the  warm  atmosphere  of  the  vales. 

Each  of  these  methods  we  will  discuss  separately. 

302.  CURVE  OF  PERPETUAL  CONGELATION.    In  Art. 
53,  we  have  seen  that  a  point  can  be  reached  in  every 
latitude,  where  moisture,  once  frozen,  always  remains 
so.     An  imaginary  line  passing  through  these  points, 
and  extending  from  pole  to  pole,  forms  what  is  termed 
the  curve  of  perpetual  congelation,  which  possesses  the 
peculiar  figure  shown  in  the  annexed  cut. 

Does  the  electric  theory  agree  with  facts  7 

What  are  Professor  Olmsted's  views  in  regard  to  electricity  J 

Whence  comes,  according  to  his  theory,  the  cold  of  the 

In  what  three  ways  may  hailstorms  arise  1 

What  is  the  curve  of  congelation  ? 

r  T. 


126 


AQUEOUS    PHENOMENA. 

Fig.  17. 


f\12,000 

10,000 
8,COO 


6,000 

\       4,000 
V        2,000 


90°  80°  70°  60°  50°  40°  30°  20°  10°  0°  10°  20°  30°  40°  50°  60°  70°  80°  90° 

CURVE  OF  PERPETUAL  CONGELATION. 

303.  The  heights  of  the  curve  from  the  surface  of  the- 
globe  vary  but  little  from  the  equator  to  Lat.  30° ;  but 
from  30°  to  60°  the  change  is  very  great,  and  the  line 
rapidly  approaches  the  earth. 

The  difference  in  the  height  of  the  points  of  congela- 
tion, for  every  five  degrees  of  latitude,  is  presented  in  the 
following  table : 

Lat.  Difference  of  height  in  (bet. 

0°  to     5° 122 

5°  to  10° 388 

10°  to  15° 569 

15°  to  20° 779 

20°  to  25° 689 

25°  to  30° 1,438 

30°  to  35° 928 

35.°  to  40° 1,648 

40°  to  45° 1,358 

45°  to  50°  .     .    .     .    .     .     .  1,398 

50°  to  55° 1,348 

55°  to  60° 1,228 

60°  to  65° 1,168 

65°  to  70° 959 

70°  to  75° 809 

75°  to  80° 628 

304.  ACTION  OF  OPPOSITE  CURRENTS.    We  are  now 
to  imagine,  for  the  sake  of  illustration,  that  a  north 


Describe  its  oeculiarities. 


ORIGIN.  127 

wind,  originating  in  50°  N.  Lat.,  moves  horizontally  at 
the  rate  of  sixty  miles  per  hour,  at  an  altitude  of  ten 
thousand  feet  ;  while  a  south  wind  blows  simultaneous- 
ly from  30°  N.  Lat.  with  the  like  velocity,  and  at  the 
same  height. 

If  they  are  upon  the  same  meridian,  they  will  meet 
in  ten  hours  at  40°  N.  Lat.,  and  since  the  first  wind 
commences  its  course  at  M,  three  thousand  feet  above 
the  limit  of  constant  frost,  it  will  be  extremely  cold  ; 
while  the  south  wind  will  be  comparatively  warm,  as  it 
proceeds  from  a  region,  N,  two  thousand  feet  below  the 
boundary  of  perpetual  congelation. 

By  the  union  of  air,  thus  widely  differing  in  temper- 
ature, the  inherent  atmospheric  vapor  is  both  condensed 
and  frozen, 'and  the  central  portion  of  the  hailstone 
formed,  which,  in  its  descent  to  the  earth,  is  gradually 
enlarged  by  constant  accretions  of  frozen  moisture. 

305.  The   prevalence   of  such   opposite  currents  as 
have  just  been  supposed,  has  already  been  shown  (Art. 
222) ;  and  it  is  by  no  means  improbable  that,  in  their 
ceaseless  circuits,  there  are  times  in  which  they  encoun- 
ter each  other.     It  may  be  asked,  how  can  the  different 
winds  preserve  their  respective  temperatures,  in  traversing 
a  distance  often  degrees?  To  this  it  is  answered,  that  a 
fluid  in  motion  can  pass  through  a  fluid  of  the  same 
kind  in  repose,  and  differing  in  respect  to  heat,  without 
suddenly  changing  its  own  temperature.     The  wraters 
of  the  Gulf-stream,  flowing  through  the  North  Atlantic 
from  the  torrid  zone,  are  warmer  than  the  ocean,  even 
in  the  latitude  of  Newfoundland. 

306.  The  occurrence  of  hailstorms,  under  these  cir- 
cumstances, substantially  agrees  with  facts.     It  will  be 
seen,  by  referring  to  the  figure,  that  the  mingling  of 
opposite  winds,  at  a  lofty  elevation,  in  the  tropics,  C,  C% 
would  occasion  nothing  but  a  combination  of  warm  cur- 


Explain  the  action  of  opposite  currents. 
Why  can  the  currents  preserve  their  respective  temperatures  ? 
Show  to  what  extent  the  occurrence  of  hailstorms,  under  these  circum- 
stances, accords  with  facts. 


128  AdUEOUS    PHENOMENA. 

rents,  and  in  the  polar  climes  of  cold  currents,  A,  A*; 
in  neither  case  could  hail  be  the  result  of  the  union. 

In  the  temperate  ^regions,  the  admixture  of  warm  and 
intensely  cold  currents  can  only  be  found,  and  precisely 
within  these  limits  hailstorms  are  most  prevalent. 

Their  frequency  in  summer  is  attributed  to  the  cir- 
cumstance, that  the  opposing  winds  are  then,  most 
easily  set  in  motion  by  the  increased  energy  of  the  solar 
rays. 

307.  The  space  ravaged  by  hailstorms,  often  indicates 
the  presence  of  aerial  currents,  the  devastations   being 
frequently  confined  to  a  long  and  narrow  strip  of  coun- 
try.    Sometimes  the   storm   proceeds   in    two    parallel 
tracks,  leaving  the  intervening    region  entirely  unin- 
jured. 

Thus  a  hailstorm  once  commenced  in  the  south  of 
France  in  the  morning,  and  in  a  few  hours  reached  Hol- 
land. The  places  desolated  formed  two  parallel  paths 
from  S.  W.  to  N.  E. ;  the  length  of  one  was  435  miles  ; 
and  that  of  the  other  497  miles.  The  average  width  of 
the  eastern  track  was  five  miles,  and  that  of  the  western 
ten  ;  and  upon  the  space  comprised  between  them,  which 
was  twelve  miles  and  a  half  in  breadth,  no  hail  fell,  but 
only  a  heavy  rain. 

308.  ACTION  OF  WHIRLWINDS.    It  has  been  stated, 
(Art.  132,)  that  whirlwinds  are  not  always  vertical,  but 
frequently  inclined  towards  the  earth.     In  consequence 
of  this  position,  the  gyratory  motion  of  the  whirl  (if  its 
diameter  is  considerable)  will,  doubtless,  often  carry  up 
hot  and  humid  air  from  the  surface  of  the  earth  into 
the  higher  regions  of  the  atmosphere,  bringing  down  in 
return  large  volumes  of  cold  air  from  the  upper  strata  ; 
thus  causing  such  a  combination  as  results  in  the  pro- 
duction of  hail.    This  action  will  be  more  extensive  and 
energetic  if,  as  some  suppose,  whirlwinds  at  times  exist 
whose  axes  are  parallel  to  the  horizon. 

309.  It  must  also  be  remembered,  that  in  the  vortex 
of  the  whirlwind  the  air  is  rarefied,  and  into  this  partial 

Explain  the  action  of  whirlwinds. 


ORIGIN.  129 

void  the  cold  air  from  above  will  descend,  by  reason  of 
its  superior  weight;  while  below,  on  account  of  the 
pressure  of  the  surrounding  atmosphere,  warm  currents 
will  stream  under  the  base  into  the  vortex.  Here,  then, 
may  evidently  occur  a  union  of  hot  and  cold  air,  differ- 
ing so  greatly  in  temperature  that  the  condensed  moist- 
ure will  freeze  into  hail. 

The  cold,  arising  from  the  rarefaction  of  the  air  in  the 
centre  of  the  whirlwind,  also  contributes  to  the  forma- 
tion of  hail. 

310.  INFLUENCE  OF  HIGH  MOUNTAINS.    In  the  viciii- 
ity  of  those  lofty  mountains,  whose  peaks   are  always 
covered  with  ice  and  snow,  destructive  hailstorms  fre- 
quently occur.     The  south  of  France,  which  lies  be- 
tween the  Alps  and  Pyrenees,  is  annually  ravaged  by 
hail ;  so  great  is  the  ruin  to  the  productions  of  the  soil, 
and  especially  the  vineyards,  that  the  yearly  loss  to  the 
national  revenue  was  estimated,  by  the  Linnean  Society 
of  Paris,  at  fifty  millions  of  francs,  or  nine  millions  three 
hundred  and  seventy-five  thousand  dollars. 

In  Peru,  hail  has  been  seen  to  fall ;  and  on  the  17th 
of  August,  1830,  it  covered  the  streets  of  Mexico  to  the 
depth  of  several  inches. 

311.  That  such  phenomena  should  arise  in  these  and 
similar  localities,  is  by  no  means  surprising:  for  cold 
blasts  of  wind  descending  from  the  snowy  summits  of 
the  neighboring  mountains,  and  mingling  with  the  warm 
air  of  the  plains,  could  doubtless  occasion  these  results  ; 
and  the  existence  of  such  breezes  is  fully  established. 

312.  HAIL   IN   SOUTHERN  INDIA.     Hail  sometimes 
occurs  within  the  tropics,  even  at  a  distance  from  those 
mountain-chains  that  rise  above  the  limit  of  perpetual 
frost.    Thus  in  India,  in  16°  30'  N.  Lat.,  during  the  year 
1825,  hailstones  fell  at  Darwar,  of  the  size  of  pigeons' 
eggs ;    and  in  a  similar   storm,    which   happened  at 


In  what  localities  do  hailstorms  occur  1 

Give  instances. 

What  is  the  cause  of  hail  in  these  regions'] 

Does  hail  ever  occur  at  a  distance  from  snow-capped  mountains  1 


130  AQUEOUS    PHENOMENA. 

Trinconopoly,  in  1805,  the  stones  were  as  large  as  wal- 
nuts. 

313.  In  conclusion,  we  may  say  in  regard  to  this  sub- 
ject, that  at  present  it  is  not  fully  understood.  Much 
valuable  information  has  been  gathered,  but  hitherto  no 
theory  has  been  advanced,  which  completely  accounts 
for  all  the  facts  that  arise. 

Give  instances. 

la  this  subject  fully  understood! 


PART    IV. 

ELECTRICAL    PHENOMENA. 


CHAPTER  I. 

OF  ATMOSPHERIC   ELECTRICITY. 

1J  I.  THE  atmosphere  is  usually  electrified.  The 
means  employed  for  collecting  its  electricity  differ  ac- 
cording to  the  object  proposed;  for  we  may  desire  to 
conduct  our  investigations  at  one  time  in  the  lower 
regions  of  the  atmosphere,  at  another  in  the  highei*;  or 
the  air  may  be  explored  to  a  great  distance  horizontally. 

315  ELECTROMETERS.  For  ascertaining  the  elec- 
tric state  of  the  atmosphere  near  the  surface  of  the  earth, 
Volta's  electrometer  is  sufficient.  An  electrometer  is  an 
instrument  which  serves  to  indicate  and  measure  elec- 
tricity. The  one  just  mentioned  consists  of  a  glass  jar, 
surmounted  by  a  pointed,  metallic  rod  ;  and  to  the  lower 
end  of  the  rod,  which  enters  the  jar,  two  fine  straws  are 
loosely  attached.  The  pointed  rod,  collecting  the  elec- 
tricity from  the  air,  the  two  straws  become  similarly 
electrified  and  recede  from  each  other,  (C.  957) ;  the 
amount  of  divergence  measuring  the  intensity  of  the 
fluid. 

316.  Insulated    rods   of  iron  are  erected  for  testing 

What  is  the  subject  of  part  fourth  1 

What  of  chapter  first  ? 

What  is  the  usual  state  of  the  atmosphere  1 

Why  are  different  means  employed  for  collecting  its  electricity  7 

What  are  electrometers  1 

How  is  the  electric  state  of  the  atmosphere  near  the  earth  ascertained  1 


132  ELECTRICAL    PHENOMENA. 

the  air  at  greater  elevations.  By  means  of  its  pointed 
summit,  the  entire  conductor  becomes  charged  with 
atmospheric  electricity,  the  nature  of  which  is  easily 
determined  by  the  electrometer. 

At  the  Ke\v  Observatory,  near  London,  the  conductor 
is  a  conical  tube  of  thin  copper,  raised  sixteen  feet 
above  the  roof;  to  the  top  of  the  tube  a  lamp  is  affixed, 
its  ascending  stream  of  smoke  and  heated  air  being  an 
excellent  collector  of  electricity. 

Where  a  fixed  apparatus  is  not  at  hand,  observations 
may  be  made  by  discharging  metallic  arrows  into  the 
air,  in  the  way  hereafter  to  be  described. 

317.  Experiments  are  made  in  the  higher  regions  of 
the  atmosphere  by  the  aid  of  kites  and  balloons.     The 
string  of  the  kite  must  be  wound  with  fine  wire,  in  order 
to  convey  the  electric  fluid  from  the  sky  |  and  it  must 
also  be  insulated,  by  attaching  the  lower  end  either  to 
a  silken  cord  or  glass  pillar.     Small,  stationary  balloons 
are    sometimes   employed,    the   strings   of    which    are 
arranged  and  fastened  in  the  same  manner. 

Occasionally  meteorologists  ascend  in  balloons  for 
the  purpose  of  making  observations. 

318.  The  method  adopted  by  Mr.  Crosse,  of  Broom- 
.field,  near  Taunton,  for  exploring  the  atmosphere  in  a 
horizontal  direction,  is  the  following.     Upon   some  of 
the  loftiest  trees  on  his  estate,  strong  poles  are  firmly 
fastened,  and  a  copper  wire  extended  from  tree  to  tree ; 
its  length  was,  originally,  a  mile  and  a  quarter,  but  is 
now  about  1600  feet.     The  wire,  being  perfectly  insu- 
lated, forms  a  conductor,  conveying  the  electricity  of 
the  atmosphere  to  the  room  of  the  observer ;  where  one 
end  of   it  terminates  in  an  insulated  brass  ball,  near 
which  is  a  receiving  ball,  connected  with  the  ground. 

In  all  apparatus  for  collecting  atmospherical  electri- 
city, the  most  careful  and  certain  arrangements  should 
be  made  for  conveying  harmlessly  to  the  earth  any 
excess  that  may  accumulate. 

How  at  greater  elevations  ? 

For  what  purpose  are  kites  and  balloons  employed  1 

\Yhat  was  the  object  of  Mr.  Crosse's  apparatus  1    Describe  it. 


ATMOSPHERIC    ELECTRICITY.  133 

By  the  aid  of  the  instruments  just  described,  much 
important  knowledge  has  been  acquired  in  regard  to  the 
electric  condition  of  the  atmosphere. 

319.  ELECTRIC  CONDITION  OP  THE  ATMOSPHERE. 
In  the  ordinary  state  of  the  atmosphere,  its  electricity 
is  invariably  positive ;  but  when  the  sky  is  overcast, 
and  the  clouds  are  moving  in  different  directions,  it  is 
subject  to  great  and  sudden  variations  ;  rapidly  chang- 
ing from  positive  to  negative,  and  back  again,  in  the 
space  of  a  few  minutes.     Upon  the  first  appearance  of 
fogs,  rain,  hail,  snow  and  sleet,  the  electricity  is  gen- 
erally negative  ;  it  then  changes  to  positive,  gradually 
increasing  in  strength,  and  then  decreasing  in  the  same 
manner  ;  the  alternations  both  in  the  strength  and  na- 
ture  of  the    electricity  occurring   every  three  or  four 
minutes.     Similar  changes  are  observed   upon  the  ap- 
proach of  a  thunder-cloud. 

The  atmosphere  is  highly  electrical,  either  when  hot 
weather  succeeds  a  series  of  wet  days,  or  wet  weather 
follows  a  series  of  dry  days. 

320.  ANNUAL    VARIATION    IN    INTENSITY.      The 
electricity  of  the  atmosphere  is  stronger  in  winter  than 
in  summer,  and,  by  comparing  observations  from  month 
to  month,   a  gradual  decrease  in  intensity  is  perceived 
from   January  to  July,  but  an  increase  from   July  to 
January.     During  the  winter  the  electricity  strengthens 
with  the  cold. 

321.  DAILY  VARIATION.     At  sunrise  the  electricity 
of  the  air  is  weak,  but  as  the  day  advances,  it  increases 
in  power,  until  13  or  7  o'clock,  A.  M.  in  summer,  8  or  9 
in  spring  and  autumn,  and  10  or  12  in  winter ;  it  then 
begins  to  diminish,  and   by  2  P.  M.  is  hardly  stronger 
than  at  sunrise.     In  summer,  it  continues  to  decrease 
till  some  time  between  4  and  6  P.  M.,  and  in  winter  is 
weakest  about  5  P.  M. 

After  this  period  the  electricity  again  becomes  strong- 
state  the  facts  in  regard  to  the  electric  condition  of  the  atmosphere. 
What  is  said  respecting  the  annual  variation  in  intensity  1 
What  of  the  daily  variation  1 


134  ELECTRICAL    PHENOMENA. 

er,  advancing  in  intensity  until  about  two  hours  after 
sunset ;  when  it  once  more  begins  to  abate,  growing 
more  and  more  feeble  until  sunrise. 

Thus,  during  the  day,  there  is  a  regular  fluctuation 
in  the  strength  of  the  atmospheric  electricity ;  two  peri- 
ods occurring  when  its  intensity  is  greatest,  and  two 
when  it  is  least. 

322.  VARIATION    IN   ALTITUDE.      The  electricity 
of  the  air  increases  in  strength  with  the  altitude.     This 
is  shown  by  the  following  experiment,  made  by  Bequerel 
and  Breschet,  at  the   monastery   upon   the   Great  St. 
Bernard. 

Having  extended  upon  the  ground  a  piece  of  gummed 
silk,  ten  feet  long  and  seven  wide,  the  experimenters 
placed  upon  it  an  electrometer ;  to  this  they  attached 
one  end  of  a  silk  cord,  into  which  was  twisted  a  fine 
wire,  the  other  end  of  the  cord  being  fastened  to  an  iron 
arrow.  By  means  of  a  bow,  the  arrow  was  shot  upwards 
to  the  height  of  250  feet ;  and  as  in  its  ascent  the  elec- 
tricity of  the  air  was  gradually  collected  and  conveyed 
along  the  wire  to  the  electrometer,  the  straws  of  the 
latter  were  seen  to  diverge  more  and  more,  and  at  length 
to  strike  the  sides  of  the  glass  jar. 

When  the  cord  was  detached,  the  electricity  of  the 
straws  was  discovered  to  be  positive. 

323.  In  order  to   determine  whether  this  increased 
divergence  was  really  caused  by  the  superior  energy  of 
the  electricity  residing  in  the  higher  regions  of  the  atmos- 
phere, the  arrow  was  discharged  horizontally  to  the 
same  distance  as  before  ;  but,  as  it  speeded  on  its  course, 
no  increased  electric  action  was  manifested  by  the  elec- 
trometer. 

324.  Experiments  for  the  same  purpose  \vere  made 
by  two  celebrated  French  philosophers,  Gay  Lussac  and 
Biot,  during  their  aerial  voyage  in  1804.     From  the  car 


What  is  said  of  variation  in  altitude  7 

Relate  the  experiment  of  Bequerel  and  Breschet. 

Describe  that  of  Gay  Lussac  and  Biot. 

What  inference  Is  drawn  from  both  these  experiments  7 


ORIGIN.  135 

of  their  balloon  was  suspended  a  wire  170  feet  long1,  to 
the  lower  end  of  which  a  metallic  ball  was  attached  ; 
the  upper  end  being-  connected  with  an  electrometer  in 
the  car.  By  means  of  this  apparatus,  these  observers 
•vere  enabled  to  note  the  electrical  changes  occurring-  in 
'he  atmosphere  at  different  heights ;  and,  from  their 
Carious  observations,  arrived  also  at  the  conclusion,  that 
*he  electricity  of  the  atmosphere  was  positive,  and  in- 
leased  in  strength  with  the  altitude. 

ORIGIN. 

325.  EVAPORATION.      One  of    the  most  abundant 
sources  of  atmospherical  electricity  is  evaporation.     It 
was  shown  by  Volta,  whose  experiments  were  confirmed 
by  those  of  Saussure,  that  electricity  was  produced  when 
water  was  evaporated.     But  it  appears  from  the  late 
researches  of  Pouillet,  that  this  is  only  the  case  when 
the  water   is   not  pure,  and  chemical  decompositions 
occur.     If  distilled  water  is  evaporated,  no  electricity  is 
developed  ;    but  if  a  little  chalk,   lime,    salt,  or  other 
foreign  matter  is  dissolved  in  the  water,  the  rising-  vapor 
is  then  positively  electrified,  and  the  vessel  containing 
the  fluid  negatively. 

326.  Now  the  waters  of  the  earth  are  generally  in 
this  latter  condition,  being  seldom  pure,  and  the  vapors, 
which  are  constantly  ascending  from  the  ground,  will 
therefore    possess    positive  electricity,    and   the  earth 
negative. 

The  briny  waves  of  the  ocean  also  contribute  their 
share,  and  supply  the  air  with  a  great  amount  of  posi- 
tive electricity. 

327.  The  process  of  evaporation  advances  invisibly 
and  in  silence ;  and,   for  this  reason,  we  might  easily 
undervalue  its  agency  in  accumulating  those  vast  stores 

What  is  the  first  source  of  atmospheric  electricity  ? 

What  is  said  in  regard  to  the  experiments  of  Volta,  Saussure  and  Pouil- 
•fefJ 

In  consequence  of  evaporation,  does  the  air  become  positively  or  nega- 
tively electrified  ? 

Is  the  amount  of  electricity  thus  developed  supposed  to  be  great  1 


136  ELECTRICAL    PHENOMENA. 

of  electric  matter  which  arm  the  storm  with  such  ter- 
rific power.  But  when  we  reflect,  that  more  than  two 
hundred  millions  of  hogsheads  of  water  are  computed 
to  rise  daily  in  vapor  from  the  Mediterranean,  we  *hall 
find  no  difficulty  in  believing,  that  this  influence  is  one 
of  the  most  energetic  causes  of  atmospherical  electricity. 

328.  CONDENSATION.     Condensation,  or  the  change 
which  vapor  undergoes  when  returning  to  a  fluid  slate 
by  a  decrease  of  temperature,  is  another  fruitful  source 
of  electricity.     This  is  shown  from  the  great  amount 
of  electricity  occasioned  by  the  condensation  of  steam, 
as  it  issues  from  the  boiler  of  an  engine. 

In  one  instance,  the  steam  which  rushed  from  the 
safety-valve  of  an  insulated  locomotive,  was  found  to 
develop  seven  times  the  amount  of  electricity  produced 
by  an  electrical  machine,  having  a  plate  of  glass  three 
feet  in  diameter,  and  making  seventy  revolutions  in  a 
minute.  Machines  in  which  the  electricity  was  gene 
rated  by  steam,  have  been  constructed  of  such  power, 
that  a  spark  twenty-two  inches  long  has  been  obtained 
from  the  prime  conductor,  (C.  964,)  of  sufficient  energy 
to  inflame  shavings. 

329.  VEGETATION.      The  vegetable    kingdom    also 
supplies  the  air  with  a  great  amount  of  electricity. 

Plants  during  the  day  exhale  oxygen  gas  ;  in  the 
night,  carbonic  acid  gas — and  from  the  experiments  of 
Pouillet  it  appears  that  positive  electricity  rises  with  the 
latter  when  the  seeds  first  sprout,  leaving  the  earth  in 
which  they  are  placed  negatively  electrified.  The  same 
results  probably  occur  during  the  life  of  the  plant. 

330.  COMBUSTION.      Combustion    is    still    another 
source  of  electricity.     When  any  substance  is  burning, 
positive  electricity  escapes  from  it,  while  the  substance 

What  calculation  would  lead  us  to  this  conclusion  7 

What  is  the  second  source  1 

What  is  condensation  1 

How  is  it  shown  that  condensation  produces  electricity  1    Illustrate. 

What  is  the  third  source  of  atmospheric  electricity  1    Explain. 

What  is  the  fourth  1 

During  combustion,  does  the  air  receive  positive  electricity  or  negativel 


THUNDER-STORMS.  137 

itself  is  negatively  electrified;  the  atmosphere  is  there- 
fore the  reservoir  of  all  the  positive  electricity  originating 
in  this  manner. 

331.  FRICTION.  In  accounting-  for  the  electricity  of 
the  atmosphere,  the  effect  of  friction  is  not.  to  be  disre- 
garded. If  a  piece  of  silk  is  shaken  in  the  air,  it  be- 
comes electrified  ;  and  it  is  highly  probable,  that  when 
masses  of  air,  moving  in  contrary  directions,  encounter 
each  other,  electricity  is  developed  by  the  friction  of 
their  surfaces.  Such  will  be  the  effect,  according  to 
Kaemtz,  when  the  masses  differ  in  respect  to  moisture 
and  temperature  ;  the  warmer  then  becomes  positively 
electrified,  and  the  colder  negatively. 

The  action  of  the  wind  upon  terrestrial  objects,  as 
rocks,  buildings,  trees,  and  hills,  may  possibly  in  like 
manner  produce  electricity. 


CHAPTER  II. 

OF  THUNDER-STORMS. 

332.  GENERAL  DISTRIBUTION.  Thunder-storms 
prevail  most  in  the  torrid  zone,  and  decrease  in  fre- 
quency towards  either  pole. 

During  a  residence  of  six  years  in  Greenland,  70°  N. 
Lat.,  Gisecke  heard  the  rolling  of  thunder  but  once  ; 
and,  according  to  the  testimony  of  the  arctic  navigators, 
Scoresby,  Parry  and  others,  thunder-storms  rarely  occur 
between  the  70th  and  75th  degree  of  north  latitude ; 
and  never  beyond  the  latter  parallel.  As  respects  time, 
they  are  more  frequent  during  the  summer  months. 


What  is  the  effect  of  friction  ? 

If  two  bodies  of  air  differ  in  temperature,  in  what  manner  will  the  elec- 
tricity, developed  by  their  friction,  be  distributed  ? 
What  may  possibly  be  the  effect  of  the  friction  caused  by  wind  ? 
Of  what  does  chapter  second  treat  1 
How  are  they  distributed  in  regard  to  latitude  and  time? 


138 


ELECTRICAL    PHENOMENA. 


The  prevalence  of  these  laws  will  be  seen  from  th« 
observations  contained  in  the  following  table. 


Places.. 

Buenos  Ayres, 
Rio  Janeiro,  . 
Calcutta,    .     . 
Padua,  .     .     . 
Paris,    .     .     . 
St.  Petersburg, 

Latitude. 

Period  of 
Observation. 

No.  of  days  of 
thunder  in  the 
year. 

No  ot  days  of 
thunder  during  6 
summer-months. 

34°  30'  S. 
22°  54'  S. 
•22°  30'  N. 
45315'N. 
48°  30'  N. 
59°  56'  N. 

7  years. 
6      " 
1  year. 
4  years. 
51      " 
11      " 

23 
51 
60 
19 
14 
9 

13 
43 
45 
14 
12 
8 

333.  Thunder-storms   are  most  violent  within  the 
torrid  zone.     Here  the  play  of  the  lightning  is  inces- 
sant, and  the  crashing  bursts  of  thunder  most  terrific ; 
and  none  but  those  who  have  actually  witnessed  a  trop- 
ical tempest,  can  form  an  idea  of  its  awful  power.    Occa- 
sionally, in  the  higher  latitudes,  fierce  storms  occur,  like 
that  which  was  seen   by   Simpson   in   the  Straits  of 
Boothia.     (Art.  294.) 

334.  ORIGIN.     The  thunder-storm  is  produced  in  the 
same  manner  as  the  common  rain-storm  ;  namely,  by 
the  condensation  of  atmospheric  vapor ;  but  it  differs  in 
two  respects  ;  first,  in  the  rapidity  of  this  condensation, 
and  secondly,  in  the  accumulation  of  electricity  resulting 
therefrom. 

335.  Wo  have  seen  that  when  vapor  is  condensed, 
electricity  is  developed  (Art  328) :  the  cloud  then  in  the 
very  process  of  formation  becomes  electrified,  and  to  its 
own  electricity  is  added  that  which  collects  upon  its  sur- 
face from  the  atmosphere  ;  wheihe'-  derived  from  evapo- 
ration,  combustion,  vegetation,  friction,  or    any  other 
source. 

This  condensation  must  be  copious,  or  the  electricity 
would  be  weak ;  it  must  also  be  rapid,  else  it  will  es- 


Repeat  the  table. 

Where  are  thunder-storms  most  violent  ? 

How  does  the  thunder-storm  differ  from  the  common  rain-storm? 

Whence  is  the  electricity  of  the  thunder-cloud  derived  1 

Why  must  the  condensation  be  both  copious  and  rapid? 


THUNDER- CLOUDS.  139 

cape  too  fast  from  the  cloud,  and  never  collect  in  suffi- 
cient strength. 

336.  Thunder-storms  are  usually  attended  by  a  change 
in  the  direction  of  the  wind,  which  accounts  for  the  con- 
densation of  atmospheric  vapor  ;  indeed,  one  of  the  most 
sublime  elements  of  a  storm  of  this  nature,  is  the  conflict 
and  raging  of  opposing  currents. 

In  the  Meteorological  Register  of  Yale  College  are 
recorded  116  thunder-storms,  which  occurred  between 
1804  and  1823.  Of  this  number,  ninety-nine  were 
either  preceded  or  followed  by  an  alteration  in  the  direc- 
tion of  the  wind  ;  the  change  in  Jjfty  instances  being 
from  a  south-westerly  breeze  to  a  north-westerly. 

Since  the  air  abounds  with  vapor  when  its  tempera- 
ture is  high,  the  condensation  will  be  most  copious  if 
a  loss  of  heat  then  suddenly  takes  place.  We  there- 
fore easily  perceive  the  reason,  why  thunder-storms  are 
more  frequent  in  summer  than  in  winter,  in  low  than  in 
high  latitudes,  and  their  intensity  greatest  in  the  tropic 
climes. 

For  the  same  reason  they  happen  more  frequently 
after  mid-day  than  in  the  morning. 

337.  ELECTRICAL    STATE    OP    THUNDER-CLOUDS. 
Since  the  air  surrounding  it  is  a  non-conductor,  a  single 
thunder-cloud  floats  in  the  atmosphere  a  vast  insulated 
conductor  (C.  963) ;    its  electricity  being  spread  over 
the  surface  of  the  globules  of  which  it  is  composed,  and 
there  retained  by  the  pressure  of  the  atmosphere. 

338.  Thimder-clouds  may  be  either  positively  or  neg- 
atively electrified  ;  and   the  observations  of  Mr.  Crosse 
lead  to  the  conclusion,  that  at  times  a  cloud  of  this  kind 
is  complex,  consisting  of  a  series  of  concentric  bands  or 
zones,  alternately  positive  and  negative  ;  the  electricity 
being  weakest  at  the  edges  of  the  cloud,  and  strongest 
at  the  centre. 

How  is  this  condensation  effected  ?     What  fact  is  stated  in  proof? 
Explain  the  cause  of  the  differences  that  exist  in  the  frequency  and  via 
fence  of  thunder-storms. 

What  is  the  electric  state  of  a  single  thunder-cloud  1 
State  Mr.  Crosse's  opinion. 


140  ELECTRICAL    PHENOMENA. 

Thus  in  figure  18.,  which  rep-  Fis 

resents  a  section  of  such  a  cloud 
obliquely  seen,  P  P'  P",  (fee.,  are 
positive  zones,  N  N'  N",  &c.,  neg- 
ative, and  the  number  of  dashes 
show  the  increase  of  intensity. 

339.  ELECTRIC    ACTION    OF 
THUNDER-CLOUDS.      The    earth 
may  be  regarded  as  a  reservoir  of 
electricity :    when,    therefore,    an 
electrified  cloud  floats  near  its  sur- 
face, it  induces  the  opposite  elec- 
tricity upon  the  ground  immedi- 
ately beneath  it. 

The  cloud  may  approach  so  near,  that  the  mutual  at 
traction  of  the  two  electricities  overcomes  the  pressure 
of  the  atmosphere ;  a  union  then  occurs,  and  the  light- 
ning, at  the  same  moment,  is  seen  darting  between  the 
cloud  and  the  earth,  and  soon  after  the  rolling  of  thun- 
der is  heard. 

340.  A  similar  inductive  action  arises  between  the 
clouds  themselves  ;  for.  if  two  clouds  differently  electri- 
fied approach  each  other,  the  electricity  upon   the  near- 
est opposite  surfaces  augments  in  intensity,  and  often 
increases  to  such  a  degree  that  a  discharge  takes   place, 
the  lightning  then  flashing  from  cloud  to  cloud.    It  may 
sometimes  happen,  that  the  path  of  least  resistance  Avill 
not  be  directly  through  the  air,  but  from  the  first  cloud 
to  the  earth,  and  from   the  earth   to  the  second  cloud, 
and  under  these  circumstances  the  lightning  will  take 
the  latter  route. 

341.  RETURN-STROKE.      When    a    highly  charged 
thunder-cloud  approaches  the  earth,  it  induces,  as  al- 
ready stated,  the  opposite  kind  of  electricity  upon  the 
ground  below,  and  repels  that  of  the  same  kind.    Should 

Describe  the  electric  action  of  thunder-clouds. 
When  does  a  flash  occur? 

What  is  the  influence  of  one  cloud  upon  another  1 
Why  does  the  lightning  in  passing  from  cloud  to  cloud  sometimes  tak« 
»he  earth  in  its  course?    What  is  the  return-stroke  1 


RETURN-STROKE.  141 

the  cloud  be  extended,  and  come  within  striking  dis- 
tance, either  of  the  earth  or  of  another  cloud,  a  flash  at 
one  extremity  is  often  followed  by  a  flash  at  the  other. 
This  is  called  the  return-stroke,  which  sometimes  oc- 
curs with  such  violence  as  to  destroy  life,  even  at  the 
distance  of  several  miles  from  the  place  of  the  first  dis- 
charge. The  mode  of  action  may  be  explained  by 
means  of  the  following  figure. 

Fig.  19. 


342.  Let   D  B  represent  a  thunder-cloud,  positively 
electrified,  and  within  striking  distance  of  the  tree  A  ; 
the  cloud,  at  D,  being  near  the  summit  of  the  hill,  C. 
By  the  inductive  action  of  the  cloud,  the  positive  elec- 
tricity will  be  repelled  from  the  tree,  A,  and   the  sum- 
mit. C :  and  both  will  be  highly  charged  with  negative 
electricity,  just  before  the  flash  occurs.     The  moment 
this   happens  at  B,  the  cloud   becomes  unelectrified,  its 
inductive  action  upon  C  suddenly  ceases,  the  positive 
electricity,   which   had    been   repelled,  instantaneously 
returns,  and,  uniting  with  the  negative  electricity  at  C, 
produces  an  explosion.    If,  at  this  time,  a  person  should, 
unfortunately,  be  standing  upon  the  top  of  the  hill,  his 
death  mignt  ensue. 

343.  In  this  manner  the  following  singular  facts  have 

How  is  it  caused  1 

Illustrate  from  the  figure. 

Relate  the  instances  given  in  Art.  343. 


142  ELECTRICAL    PHENOMENA. 

been  explained,  which  happened  on  the  10th  of  July, 
17S5,  in  the  vicinity  of  Coldstream,  in  Berwickshire. 
After  a  fine  morning,  clouds  were  seen  in  the  north- 
west by  the  observer  Brydone,  at  about  eleven  o'clock. 
Between  twelve  and  one  o'clock,  the  storm  being  still 
distant,  lightnings  were  seen  darting  from  cloud  to 
cloud,  followed  by  thunders.  Immediately  after,  Bry- 
done was  startled  by  several  loud  explosions  near  his 
house,  like  the  reports  of  a  gun.  At  this  moment  two 
carts  loaded  with  coals  were  passing  by.  The  driver 
and  horses  of  the  first  were  instantly  killed,  and  the  coal 
scattered  in  all  directions,  while  the  driver  of  the  second 
wagon,  which  was  about  twenty  yards  behind,  neither 
perceived  any  lightning  nor  experienced  any  shock. 
Upon  examination,  the  hair  on  the  legs  and  bellies  of 
the  horses  was  found  to  be  singed, «nd  where  the  wheels 
rested  at  the  time  of  the  explosions,  the  tire  was  melted, 
and  two  round  holes  were  discovered  in  the  ground.  A 
quarter  of  an  hour  before  this  event,  and  at  a  spot  nearly 
a  mile  and  three-quarters  distant,  a  shepherd  of  the 
name  of  Bell  perceived  a  lamb  suddenly  fall,  while  a 
flame  passed  before  his  face.  Upon  raising  the  lamb 
he  found  it  to  be  dead.  A  woman,  who  was  cutting 
grass  upon  the  bank  of  the  Tweed,  felt  a  violent  shock 
upon  the  soles  of  her  feet,  and  was  thrown  to  the 
ground. 

During  a  storm  which  happened  near  Manchester,  in 
June,  1835,  loud  discharges  were  heard  at  different 
points  of  a  road,  like  the  reports  of  a  pistol,  and  electric 
flashes  distinctly  seen  ;  a  person  is  said  to  have  been 
killed  at  this  time,  by  an  explosion  under  his  right  foot. 

344.  HEIGHT  OF  THUNDER-STORMS.  Though  thun- 
der-storms prevail  in  the  lower  regions  of  the  atmos- 
phere, they  have  often  been  seen  at  a  very  great  alti- 
tude. A  storm,  observed  by  Kaemtz,  amid  the  moun- 
tains of  Switzerland,  rose  to  the  height  of  more  than 
10.000  feet,  and  the  dwellers  in  the  vale  of  Chamouni 
assured  him,  that  storms  frequently  swept  over  the 

What  is  said  respecting  the  height  of  thunder-storms  1 


LIGHTNING.  143 

summit  of  Mont  Blanc.  On  the  peaks  of  the  Cordille- 
ras, a  violent  thunder-storm  was  encountered  by  La 
Condamine  and  Boguer,  at  an  elevation  of  even  16.000 
feet.  Vitrified  rocks  have  at  times  been  discovered  at 
lofty  heights,  and  as  this  change  is  supposed  by  some  to 
have  been  effected  by  lightning,  they  have  sought  to 
determine  the  altitude  of  thunder-storms  from  facts  of 
this  kind.  The  reasoning,  however,  is  inconclusive,  for 
these  vitrifications  may  be  owing  to  other  causes,  and 
were  we  even  to  grant  that  they  are  produced  by  light- 
ning, the  case  is  by  no  means  proved ;  since  a  flash 
sometimes  passes  between  the  clouds  and  the  earth, 
when  the  former  are  below  the  point  that  is  struck. 

Thus,  on  the  first  of  May,  1800,  a  church  situated 
on  Mount  St.  Ursula,  a  lofty  peak  inStyria,  was  struck  ; 
and  seven  persons  were  killed  by  a  flash  of  lightning 
darting  upwards  from  a  thunder-storm  below. 

345.  From  the  observations  of  Peytier  and  Hossard 
among  the  Pyrenees,  it  appears,  that  the   upper   and 
lower  surfaces  of  thunder-clouds  bear  no  resemblance 
to  each  other,  for  while  the  latter  are  perfectly  level,  the 
former  are  broken  and  uneven,  presenting  the  appearance 
of  mountains  and  ridges  ;  whence,  during  seasons  of  great 
heat,  lofty  peaks  and  pinnacles  of  clouds  shoot  far  up 
into  the  sky. 

LIGHTNING. 

346.  ORIGIN.     "When  a  portion  of  air  is  subjected  to 
a  very  sudden  and  powerful  compression,  a  spark  is 
elicited  (Art.  551) :  that  electricity  produces  such  a  com- 
pression can  be  proved  by  experiment,  and  to  the  ener- 
getic condensation  of  the  atmosphere  before  the  electric 
fluid,  in  its  rapid  progress  from  point  to  point,  is  at- 
tributed the  vivid  flashes  that  illumine  the  stormy  sky. 

347.  KINDS.     Lightning  has  been  divided  by  Arago 
into  three  kinds,  principally  distinguished  by  their  form 


What  did  Peytier  and  Hossard  observe  1 

What  is  the  cause  of  lightning? 

Into  how  many  kinds  has  it  been  divided  by  Arago  1 


144  ELECTRICAL    PHENOMENA. 

viz.,  zigzag-lightning,  sheet-lightning,  and  ball-light 
ning. 

348.  ZIGZAG-LIGHTNING.     This   kind  is  so   called 
from  the  peculiarity  of  its  figure,  which  is  thus  explain- 
ed.    As  the  electricity  passes  through  the  atmosphere, 
the  air  is  supposed,  at  length,  to  be  so  powerfully  com- 
pressed before  it,  that  a  great  resistance  is  presented, 
and  the  electric  fluid  then  finds  an  easier  route  in  some 
other  direction.     In  this  it  proceeds,  until  it  once  more 
meets  with  a  like  opposition,  and  is  compelled  again  to 
change  its  course  ;  and  thus  it  continues  glancing  from 
side  to  side,  until  at  last  it  reaches  the  place  it  seeks. 

Zigzag-lightning  appears  as  a  narrow,  jagged  line  of 
intensely  vivid  light,  traversing  space  with  extreme 
velocity.  On  account  of  the  unequal  conducting  power 
of  different  portions  of  the  atmosphere,  the  flash  some- 
times divides,  branching  out  in  several  different  direc- 
tions ;  the  lightning  is  then  said  to  be  forked.  A  divi- 
sion into  three  distinct  lines  is  of  rare  occurrence;  but 
even  more  have  been  seen,  for  Kaemtz  beheld,  at  Halle, 
in  June,  1834,  a  flash  of  lightning  which  threw  out  nu- 
merous branches  at  the  sides ;  the  whole  presenting  the 
figure  of  a  spine,  with  its  supporting  ribs. 

It  is  said  that  zigzag-lightnings  usually  pass  between 
the  clouds  and  the  earth,  seldom  flashing  from  cloud  to 
cloud. 

349.  SHEET-LIGHTNING.     This   kind  is   the  most 
common,  and  appears  during  a  storm  as   a  diffuse  glow 
of  light,  illuminating  the  edges  of  the  clouds;  and  at 
times  breaking  out  from  the  central  mass.     When  it 
occurs,  the  clouds  are  said  to  open.     The   flashes  of 
sheet-lightning  often  follow  each  other  in  rapid  succes- 
sion, for  the  space  of  many  hours  ;  their  intensity  is  by- 
no  means  great,  and  the  thunder  which  attends  them  is 
low  and  distant. 

.350.  BALL-LIGHTNING.     Lightning  of  this  class  is 

What  are  they  ? 

To  what  is  the  peculiar  figure  of  zigzag-lightning  owing  7 

What  is  its  appearance  7 

Describe  sheet-lightning.     Describe  ball-lightning. 


BALL-LIGHTNING.  145 

extremity  rare,  and  so  singular  are  its  attendant  phe- 
nomena, that  we  might  well  doubt  its  existence.  w?re 
not  the  instances  of  its  occurrence  fully  authenticated, 
In  a  storm  that  happened  at  Steeple  Aston,  Wiltshire, 
in  1772,  the  Rev.  Messrs.  Pitcairne  and  Wain  house, 
while  in  the  vestry  of  the  church,  saw  suddenly  before 
them,  at  the  distance  of  a  foot,  and  at  about  their  o\vn 
height  from  the  floor,  a  ball  of  fire,  nearly  the  size  of  a 
marts  fist,  surrounded  by  a,  black  smoke.  It  burst  with 
an  explosion  like  the  discharge  of  several  cannon 
Pitcairne  was  dangerously  wounded,  and  his  person  and 
clothes  showed  the  usual  marks  of  lightning. 

'  During  a  thunder-storm  that  occurred  in  1809,  at 
Newcastle  on  Tyne,  the  house  of  David  Sutton  was 
struck  :  the  lightning  descending  the  chimney.  After 
the  explosion,  several  persons  who  were  assembled  in  a 
room,  saw  at  the  door  a  globe  of  fire,  which,  after  re- 
maining stationary  for  some  time,  advanced  into  the 
middle  of  the  room,  where  it  burst  into  fragments,  with 
a  report  like  a  rocket. 

351.  On  the  fourth  of  November,  1749,  in  42°  48'  N. 
Lat.,  2°  W.  Long.,  the  crew  of  the  ship  Montague  be- 
held, a  little  before  noon,  and  beneath  an    unclouded 
sky,  a  globe  of  bluish  fire,  like  a  millstone,  rolling  rapidly 
upon  the  sea.     At  a  short  distance  from  the  vessel,  it 
rose    perpendicularly  from   the  water,  and    struck    the 
masts  with  an  explosion  louder  than  the  discharge  of  a 
hundred  cannon.     Five  sailors  were   thrown  senseless 
upon  the  deck,  one  of  whom  was  severely  burned. 

In  the  midst  of  a  storm  in  Scotland,  two  globes  of 
fire,  connected  together  like  chained  cannon-shot,  were 
seen  by  a  Mr.  Lumsden,  passing  through  the  sky 
revolving  one  about  the  other,  and  striking  at  last  upon 
the  summit  of  a  hill.  Philosophers  have  not  yet  been 
enabled  to  account  for  lightning  of  this  description;  it 
.las,  however,  been  supposed  to  arise  from  an  uninter- 
miitcd  discharge'  of  electricity. 

352.  HEAT-LIGHTNING.      It  not   unfrequently  Lip- 
Relate  instances.    How  is  ball-lightning  supposed  to  arise? 

What  is  heat-lightning? 

7 


146  ELECTRICAL    PHENOMENA. 

pens,  daring  the  serene  evenings  of  summer,  that  the 
horizon  is  illumined  for  many  hours  with  successive 
flashes  of  light,  unattended  with  thunder.  This  is  called 
heat-lightning,  and  has  much  perplexed  meteorologists. 
It  is  affirmed  by  some,  that  this  illumination  is  the 
reflection  from  the  atmosphere  of  the  lightnings  of  re- 
mote storms  ;  the  storms  themselves  being  so  far  dis- 
tant, that  their  thunders  cannot  be  heard.  Others  assert, 
that  during  warm,  sultry  weather,  when  the  air  is  highly 
rarefied,  its  pressure  upon  the  clouds  is  so  much  dimin- 
ished, that  the  electric  fluid  can  never  accumulate  upon 
their  surface  beyond  a  certain  point,  when  it  escapes 
in  noiseless  flashes  to  the  earth. 

353.  Multiplied  observations  have  proved,  that  heat- 
lightning    generally  originates  in   the   first-mentioned 
cause ;  but  the  instances  are  by  no  means  rare,  when 
silent  flashes  of  electric  light  play  between  the  earth 
and  the  clouds.     These  cases  occur  when  the  weather 
is  sultry,  the  air  being  then  both  rarefied  and  moist ; 
two  conditions  which  lessen  its  non-conducting  power ; 
the  atmosphere  thus  becomes   an  imperfect  conductor 
between  the  clouds  and  the  earth,  which  are  in  opposite 
electrical  states,  and  opposes  just  sufficient  resistance  to 
the  passage  of  the  electric  fluid  as  to  render  it  visible. 

354.  VELOCITY  OF  LIGHTNING.    By  a  very  ingenious 

Ciece  of  apparatus,  Prof.  Wheatstone,  of  King's  Col- 
5ge,  London,  has  been  enabled  to  show  that  the  dura- 
tion of  a  flash  of  lightning  is  less  than  the  thousandth 
'fart  of  a  second,  and  Arago  has  demonstrated   that  it 
does  not  exceed  the  millionth  part. 

Now  the  duration  of  a  flash,  is  the  time  it  wcupies*  in 
traversing  the  space  between  two  clouds,  or  between  a 
cloud  and  the  earth  ;  if  we  then  estimate  this*  distance 
to  be  equal  sometimes  to  a  quarter  of  a  mile,  which  is 
a  low  computation,  the  velocity  of  lightning,  in  such 
cases,  according  to  Arago,  could  not  be  less  than  '^50  000 
miles  per  second.  The  electricity  developed  by  the 


How  does  it  originate1] 

What  is  said  in  regard  to  the  velocity  of  lightning? 


EFFECTS    OF    LIGHTNING.  147 

electrical  machine,  has  heen  shown  by  anothe*  beau- 
tiful contrivance  of  Prof.  Wheatstone,  to  possess  a  speed 
of  288,000  miles  per  second :  the  rapidity  of  lightning  is 
probably  not  less. 

The  preceding  remarks  apply  only  to  lightnings  of  the 
first  and  second  class.  Ball-lightnings,  on  the  contrary, 
often  move  slowly,  and  are  visible  for  many  seconds. 

355.  COLOR.     When   thunder-clouds   are     near   the 
earth,  the  flashes  are  of  a  brilliant  white  ;  but  when  the 
storm  is  high,  and  the  lightnings  play  through  a  rarefied 
atmosphere,  their  color,  approaches  to  violet.     A  spark 
of  electricity  assumes  the  same  hue,  when  it  is  made  to 
pass  through  the  exhausted  receiver  of  an  air-pump. 

356.  EFFECTS  OF  LIGHTNING.     These  are  precisely 
similar  to  those  of  common  electricity  in  kind,  though 
far  exceeding  them  in  degree.     Life  is  destroyed  by  the 
shock,  the  stoutest  trees  shivered  to  pieces,  ponderous 
weights  displaced,  combustibles  inflamed,  metals  soft- 
ened and  fused,  sand  vitrified,  and  iron  and  steel  ren- 
dered magnetic.     It  is  needless  to  multiply  instances  in 
proof  of  these  particular  points,  but  a  few  cases  may 
tend  to  impress  them  upon  the  mind. 

357.  On   the  night  of  the  21st  of  June,  1723,  a  tree 
in  the  forest  of  Nemours  was  struck  by  lightning.     The 
trunk  was  split  into  two  fragments,  one  seventeen  feet 
long,  the  other  twenty-two  ;  and  though  the  first  required 
four  men  to  lift  it,  and  the  second  eight,  yet  both  of 
them  were  hurled  to  a  distance  of  seventeen  yards.    On 
the  6th  of  August,  1809,  a  flash  of  lightning  struck  a 
house  at  Swinton,  near  Manchester.     The  wall  of  a 
building  attached  to  the  house  was  loosened  from  its 
foundation  a  foot  below  the  ground,  and  raised   in   a 
mass  to  the  surface,  still  maintaining  its  upright  posi- 
tion ;  one  end  of  it  was  moved  nine,  and  the  other  four 
feet  from  its  original  place.     The  wall  thus  moved  was 
eleven  feet  high  and  three  feet  thick,  and  contained  7000 
bricks,  which,  exclusive  of  the  mortar,  were  estimated  to 
weigh  nearly  twenty-six  tons. 

What  of  its  color  1    What  of  its  effects  ? 


148  ELECTRICAL    PHENOMENA. 

On  the  20th  of  April,  1807,  at  Great  Mouton,  in  Lan- 
cashire, a  windmill  was- struck  by  lightning;  the  fluid 
passed  along  a  large  iron  chain,  the  links  of  which 
were  so  softened,  that  by  their  own  weight  they  became 
welded  together ;  and  the  chain  was  converted  into  an 
inflexible  bar  of  iron. 

In  Sept.  1845,  a  house  at  New  Haven,  Ct.,  was  struck 
during  a  thunder-storm.  Several  articles  of  steel  were 
rendered  magnetic,  and  a  razor,  lying  in  a  case  near  the 
spot  where  the  lightning  entered,  was  found  capable  of 
sustaining  a  key,  weighing  half  an  ounce. 

358.  FULGURITES.     When  a  flash  of  lightning  falls 
upon  sand,  its  path  below  the  surface  is  often  marked 
by  &  fulgurite,  so  called   from  the  Latin  word  fulgur, 
lightning.    It  is  a  tube  composed  of  sand,  vitrified  by  the 
action  of  the  lightning.     Fulgurites  were  first  discov- 
ered in  Silesia,  in  1711,  and  specimens  were  forwarded 
to  the  museum  at  Dresden,  where  they  are  still  preserved  : 
they   have    since    been    found   in    great    numbers,    in 
Germany,   England,  and  amid  the  sands  of  Bahia,  in 
Brazil. 

The  fulgurite  is  winding  in  its  form,  often  throws  out 
lateral  spurs  or  brandies,  and  contracts  in  size  towards 
the  lower  extremity,  which  usually  terminates  at  a  spring 
of  water,  or  in  some  substance  that  is  a  good  conductor 
of  electricity.  . 

359.  These  tubes  are  generally  hollow,  the  interior 
surface  being  coated  with  a  brilliant  glass.     Their  di- 
ameters vary  horn  four-hundredths  of  an  inch,  to  three 
inches  and  a  half,  and  the  thickness  of  their  sides  from 
one-fiftieth  of  an  inch,  to  nearly  an  inch. 

The  branches  of  the  fulgurite,  differ  in  length  from 
three  quarters  of  an  inch  to  a.  foot,  but  the  main  tube 
often  extends  to  the  depth  of  many  yards.  Several  of 
considerable  length,  which  had  been  taken  from  the 
sandy  plains  of  Silesia,  were  exhibited  at  London,  some. 

Give  instances.     What  are  fulgurites  1 
Where  have  they  been  discovered  1 
What  is  their  form  ? 
State  their  dimensions. 


VOLCANIC   LIGHTNING.  1    9 

years  ago,  by  Dr.  Fiedler,  of  Germany.    One,  discovered 
at  Paderborn,  in  Westphalia,  was  forty  feet  long: 

360.  That  these  tubes  are  really  produced  by  light- 
ning1, has  been  proved  by  actual  observation.  A  num- 
ber of  sailors,  being1  upon  the  isle  of  Amrum,  in  Den- 
mark, saw  a  flash  of  lightning  fall  upon  the  sand  ;  upon 
examining  the  spot,  they  found  a  fulgurite:  a  similar 
circumstance  happened  on  the  borders  of  Holland. 
Savart  and  others  have  obtained  artificial  fulgurites, 
by  passing  powerful  electric  sparks  through  powdered 
glass,  and  a  mixture  of  sand  and  salt ;  tubes  were  thus 
formed  an  inch  in  length,  and  the  tenth  of  an  inch  in 
thickness,  the  inner  diameter  being  the  twenty-fifth  of 
an  inch. 

381.  VOLCANIC-LIGHTNING.  The  clouds  of  smoke, 
ashes,  and  vapor,  that  issue  from  volcanoes-during  their 
eruption,  are  the  scene  of  terrific  lightning  and  thunder. 
Pliny  the  younger,  in  his  letters  to  Tacitus,  mentions 
the  lightning  that  was  seen  above  Vesuvius,  during  its 
eruption,  in  the  year  79,  A.  D.  In  that  which  occurred 
in  1767,  the  inhabitants  at  the  foot  of  the  mountain  as- 
sured Sir  William  Hamilton,  that  they  were  more  terri- 
fied at  the  lightning  which  flashed  around  them,  than 
by  the  burning  lava,  and  all  the  other  attendant  dangers. 

During  the  eruptions  of  the  same  mountain  in  1779 
and  1794,  there  appeared,  in  the  midst  of  the  dark  vol- 
canic clouds,  globes  ofjire,  which,  bursting  like  bomb- 
shells, darted  on  every  side  vivid  flashes  of  zigzag-light- 
ning. In  the  latter  eruption  were  heard  loud  and  con- 
tinued peals  of  thunder. 

362.  The  cause  of  volcanic-lightning  is  found,  in  the 
rapid  condensation  of  the  vast  volumes  of  heated  vapor, 
which  are  carried  up  from  the  crater  of  the  volcano  into 
the  higher  and  colder  regions  of  the  atmosphere. 

In  like  manner,  in  the  midst  of  water-spouts   and 


How  is  it  known  that  they  are  actually  caused  by  lightning? 
In  what  manner  have  they  been  artificially  made  ? 
Relate  the  instances  given  of  volcanic  lightning  and  thunder 
How  are  volcanic  lightnings  caused  1 


150  ELECTRICAL    PHENOMENA. 

whirlwinds,  an  abundant  condensation  of  vapor  suddenly 
occurs,  which  frequently  develops  such  an  amount  of 
electricity,  that  the  lightning  here  displays  itself  in  all 
its  fearful  energy. 

363.  THUNDER.      In  consequence  of  the  lightning- 
passing  through   the  atmosphere  with  an  amazing  ve- 
locity, it  leaves  a  void  space   behind  it.  into  which   the 
surrounding  air  instantly  rushes,  with  a  'loud  report. 
This  noise  is  thunder. 

When  the  lightning  is  near  the  observer,  the  report  is 
sharp  and  quick,  but  when  at  a  distance,  it  is  long  and 
rolling. 

364.  The  rolling  of  thunder  is  frequently  occasioned 
by  the  reverberations  of  the  sound,  from   clouds  and 
adjacent,  mountains  ;  but  this  is  by  no  means  always  the 
case.     WheH  the  lightning-flash  darts   to  a  great  dis- 
tance, such  is  its  velocity,  that  the  thunder  may  be 
considered  as  occurring  at  every  point  of  the  flash  at  the 
same  time.    But  sound  has  a  progressive  motion  of  1142 
feet  per  second,  and  all  the  thunder  will  not  reach  the 
ear  at  the  same  instant.    It  will  be  first  heard  from  the 
nearest  point,  in  the  path  of  the  flash,  and  later  and  later 
from  points  more  remote  ;  and  the  combined  effect  will 
be  a  continued  peal. 

The  zigzag  form  of  the  flash,  and  its  division  into 
several  streams,  is  regarded  by  Herschel  as  affording  an 
adequate  explanation  for  all  the  changes  that  occur  in 
the  sound  of  the  thunder-peal. 

365.  The  time  that  elapses  between  the  lightning  and 
the  thunder,  enables  us  to  form  an  estimate  of  the  dis- 
tance of  the  former,  which  is  a  little  more  than  a  mile 
for  every  jive  seconds.      This  interval  usually  varies 
from  three  to  sixteen  seconds ;  but  cases  have  occurred, 
where  it  has  amounted  to  fifty,  and  even  seventy-two 
seconds. 

366.  IDENTITY  OF  LIGHTNING  AND  ELECTRICITY. 

What  is  the  cause  of  thunder? 

How  is  its  rolling  occasioned  1 

How  can  we  estimate  the  distance  of  lightning? 

How  great  an  interval  of  time  sometimes  occurs  1 


LIGHTNING    AND    ELECTRICITY.  151 

The  resemblance  between  lightning  and  electricity  was 
noticed  by  the  earlier  electricians,  Wall,  Grey,  and  Nol- 
let, ;  but  their  identity  was  first  established  by  Dr.  Frank- 
lin. The  strong  points  of  similarity  which  convinced 
him  of  this  fact,  were  the  following. 

1st.  Lightning  and  the  electric  spark  are  both  zig- 
zag inform, 

2d.  Lightning  strikes  trees,  chimneys,  spires,  masts 
of  vessels,  mountains  and  elevated  points  upon  the  sur 
face  of  the  earth.  Electricity  is  likewise  attracted  by 
pointed  bodies. 

3d.  Both  choose  the  best  conductors. 

kth.  Both  ignite  combustibles. 

5th.  Both  fuse  metals. 

6th.  By  the  action  of  each,  a  bad  conductor  is  shiv 
ered'when  struck. 

7th.  Lightning  reverses  the  poles  of  a  magnet,  and 
renders  iron  'magnetic.  Electricity  does  the  same. 

tith.  Animal  life  is  destroyed  by  each. 

$th.  Blindness  is  produced  by  both. 

368.  Franklin,  however,  did  not  stop  here.     He  re- 
solved to   test  the  truth  of  his  reasoning,  by  drawing 
lightning  from  the  clouds,  and  in  June,  1752,  made  the 
hazardous  experiment  in  the  vicinity  of  Philadelphia. 

369.  FRANKLIN'S   EXPERIMENT.     Having  made  a 
kite,  by  tying  the  corners  of  a  large  silk  handkerchief  to 
tbe  ends  of  two  light  strips  of  cedar  that  crossed  each 
other,  and  placed  upon  it  a  pointed  iron  wire  connected 
with  the  string,  Franklin  went  out  into  a  field  upon  the 
approach  of  a  thunder-storm,  accompanied  by  his  son. 
When  the  kite  was  raised,  he  attached  a  key  to  the  lower 
end  of  the  hempen  string ;  to  the  key  one  end  of  a  silk 
ribbon  was  now  tied,  the  other  being  fastened  to  a  post. 
The  kite  was  thus  insulated,  and  the  experimenter,  for 
a  considerable    time,   awaited   the   result  with   intense 
solicitude.     A  dense  cloud  passed  over,  but  no  indica- 


By  whom  was  the  identity  of  lightning  and  electricity  first  established? 
What  points  of  similarity  did  Franklin  observe  ? 
Relate  Franklin's  experiment. 


152  ELECTRICAL    PHENOMENA. 

tions  of  electricity  appeared  upon  the  string  ;  when,  just 
as  Franklin  began  to  despair  of  success,  he  beheld  die 
loose  fibres  of  the  cord  starting  asunder,  and  immedi- 
ately presenting  his  knuckle  to  the  key  he  received  an 
electric  spark.  The  rain  now  descending,  increased  the 
conducting  power  of  the  string,  and  vivid  electric  sparks 
issued  from  the  key  in  great  abundance.  By  means  of 
the  lightning  thus  obtained,  all  the  common  electrical 
experiments  were  performed,  and  the  identity  of  light- 
ning and  electricity  thus  indubitably  proved. 

370«  ROMAS'  EXPERIMENT.  No  sooner  was  this 
wonderful  discovery  made  know'ii,  than  men  of  science 
were  eager  to  repeat  the  experiment. 

With  a  kite  eleven  feet  hi^h  and  three  feet  wide, 
Romas  obtained  in  France  the  most  brilliant  and  a.ston- 
ishing  results.  In  one  instance,  when  the  kite  was 
raised' during  a  storm,  such  an  accumulation  of  electrici- 
ty occurred,  that  streams  of  electric  fire  nine  or  ten  feet 
long,  and  an  inch  in  thickness,  flashed  spontaneously 
from  the  string,  with  reports,  like  those  of  a  pistol. 
thirty  streams  of  this  magnitude  burst  forth  in  the 
space  of  an  hour,  without  counting  a  multitude  of  others, 
seven  feet  in  length. 

371.  RICHMAN'S  DEATH.  That  such  experiments 
are,  however,  attended  with  great  danger,  unless  every 
precaution  is  strictly  observed,  is  proved  by  the  unfor- 
tunate death  of  I?rof.  Richman,  of  St.  Petersburg,  who 
w;is  killed  by  lightning,  on  the  6th  of  August,  1753.  lie 
had  erected,  upon  the  top  of  his  house,  an  iron  rod  from 
which  proceeded  a  chain  that  entered  his  stud}'.  The 
whole  apparatus  was  entirely  insulated.  On  the  day  in 
question  while  examining  the  electrometer,  as  a  thun- 
der-storm was  approaching,  a  large  globe  of  blue  fire 
flashed  from  the  conductor  to  his  head,  instantly  depriv- 
ing him  of  life. 

Relate  Roma«'  experiment. 

What  error  did  Richman  commit  in  the  construction  of  his  apparatus'? 


LIGHTNING-ROD.  153 

LIGHTNING-ROD. 

372.  The  invention  of  the  lightning-rod  for  the  pro- 
tection of  buildings  was  the  fruit  of  the  brilliant  discov- 
ery of  Franklin.     Even   before  his  decisive  experiment 
he  had  been  led  to  suppose,  from  the  analogies  existing 
between  lightning  and  electricity,  that  pointed  metallic 
rods  might  possibly  disarm  the  thunder-cloud  of  its  ter- 
rific power. 

373.  In  order  that  the  lightning-rod,  or  conductor, 
may  afford  an  effectual  protection,  regard  must  be  had 
to  the  material  of  which  it  is  made,  its  size,  and  the 
mode  of  erection. 

374.  MATERIAL.     Wrought  iron  is  usually  employed, 
and  forms  a  good  conductor ;  but  copper  is  preferable, 
inasmuch  as  it  is  less  liable  to  be  corroded  or  fused,  and 
possesses  a  greater  conducting  power. 

375.  SIZE.     The  rod,  if  made  of  iron,  should  be  three- 
quarters  of  an  inch  in  diameter,  and  its  upper  extrem- 
ity should  terminate  in  one  or  more  points.     Each  of 
these  points  (which  are  usually  three  in  number)  ought 
to  be  capped  with  some  metal  which  does  not  rust,  as 
silver,  gold,  or  platina  ;    for  the  conducting  power  of 
the  points,  if  made  of  iron,  would  be  weakened  by  the 
rust. 

376.  MODE  OF  ERECTION.     The  rod  should  be  con- 
twmous  from  the  top  to  the  bottom  ;  an  entire  metallic 
communication  existing  throughout  Us  whole  length. 
This  law  is  violated,  when  the  joints  of  the  several  parts 
that  form  the  conductor  are  imperfect,  and  the  whole  is 
loosely  put  together.     The  parts  may  be  screwed  one 
into  the  other:  or  the  rod  may  be  formed  of  wires  twist- 
ed together. 

377.  The  conductor  should   be  fastened  to  the  build- 
ing by  wooden  supports,   but  if   masses  of  metal,  as 


By  whom  was  the  lightning-rod  invented  1 

To  what  particulars  must  attention  be  directed,  that  the  lightning  -rod 
may  afford  an  effectual  protection  1 
What  is  said  in  regard  to  the  material  1 
To  the  size  1    To  the  mode  of  erection  ? 

T 


154  ELECTRICAL    PHENOMENA. 

leaden  pipes  and  troughs,  are  connected  with  the  build- 
ing, it  is  best  to  attach  them  to  the  rod  by  strips  of 
metal ;  for,  unless  this  is  done,  lightning  may  pass  from 
the  rod  to  the  metal,  and  enter  the  edifice,  especially  if 
the  rod  is  in  any  way  defective.  By  adopting  the  above 
precaution,  the  metallic  masses  are  made  a  part  of  the 
conductor,  and  if  the  lightning  strikes  them,  it  is  con- 
veyed through  the  rod  to  the  earth. 

378.  The  lower  end  of  the  rod  should  be  divided  into 
two  or  three  branches,  so  bent  as  to  pass  away  from 
the   building ;    and   it    is   highly  essential   that    these 
branches  should  extend  so  far  below  the  surface  of  the 
ground,  as  to  reach  either  water  or  a  permanently  moist 
stratum  of  earth.     The  rod  should  be  surrounded  with 
powdered  charcoal,   which  at  once  preserves  the  iron 
from  rust,  and  facilitates  the  passage  of  electricity  be- 
tween the  metal  and  the  earth,  in   consequence  of  its 
conducting  power.     For  the  same  reason,  the  conductor 
should  be  painted  with  black  paint,  made  of  charcoal. 

379.  EXTENT  OF  PROTECTION.    According  to  the  in- 
vestigations of  M.  Charles,  the  lightning-rod  protects  the 
space  around  it  to  a  distance  equal  to  twice  its  height. 
Thus,  if  the  conductor  extends  ten  feet  above  the  sum- 
mit of  a  house,  it  affords  protection  to  a  circular  space 
forty  feet  in  diameter  ;  the  rod  being  in  the  centre. 

The  experience  of  nearly  one  hundred  years  has 
shown  that,  where  the  above  rules  and  precautions  are 
observed,  an  effectual  security  has  been  provided  against 
the  effects  of  lightning ;  so  far  as  human  means  can 
avail  to  disarm  the  elements. 

380.  It  is  an  error  to  suppose  that  conductors  attract 
the  lightning  towards  the  building  upon  which  they  are 
erected.     They  simply  direct  the  course,  and  facilitate 
the  passage  of  the  electricity  between   the  clouds  and 
the  earth,  when   a  discharge    must    inevitably    occur, 
where  the  building  is  situated. 

How  great  a  space  is  protected  by  a  lightning-rod  ? 
Huve  we  any  proof  of  the  utility  of  lightning-rods'? 
Is  a  building  more  or  less  liable  to  be  struck  when  furnished  with  a  good 
conductoi  ? 


ELECTRIC    FOGS.  155 

I(  is  indeed  highly  probable,  that  a  silent  and  gradual 
discharge  of  a  thunder-cloud,  is  often  effected  by  the 
points  of  the  rod,  and  an  explosion  thus  prevented, 
This  is  the  opinion  of  Arago,  who  expressly  states,  that 
"  lightning-rods  not  only  render  strokes  of  lightning 
inoffensive,  but  considerably  diminish  the  chance  of  a 
building-  being  struck  at  all." 

381.  ELECTRIC  FOGS.  Fogs  are  at  times  highly 
electrical ;  a  most  extraordinary  instance  is  thus  related 
by  Mr.  Crosse,  of  Broomfield,  whose  apparatus  has  al- 
ready been  described.  "  Many  years  since  I  was  sitting 
in  my  electrical  room,  on  a  dark  November  day,  during 
a  very  dense,  driving  fog  and  rain,  which  had  prevailed 
for  many  hours,  sweeping  over  the  earth,  impelled  by  a 
south-west  wind.  I  had  at  this  time  1,600  feet  of  wire 
insulated,  which  crossing  two  small  valleys,  brought 
the  electric  fluid  into  my  room.  From  about  8  o'clock 
in  the  morning  until  four  in  the  afternoon,  not  the  least 
appearance  of  electricity  \vas  visible  at  the  atmospheric 
conductor,  even  by  the  aid  of  the  most  delicate  tests. 
Having  given  up  the  trial  of  further  experiments  upon 
it,  I  took  a  book  and  occupied  myself  with  reading, 
leaving  by  chance  the  receiving  ball  upwards  of  an  inch 
from  the  ball  in  the  atmospheric  conductor.  About  four 
o'clock  in  the  afternoon,  while  I  was  still  reading,  I  sud- 
denly heard  a  very  strong  explosion  between  the  two 
balls,  and  shortly  after  many  more  took  place,  until  they 
became  one  uninterrupted  stream  of  explosions,  which 
died  away  and  recommenced  with  the  opposite  electri- 
city in  equal  violence.  The  stream  of  fire  was  too  vivid 
to  look  at  for  any  length  of  time,  and  the  effect  was 
most  splendid,  and  continued  without  intermission,  save 
that  occasioned  by  the  interchange  of  electricities,  for 
upwards  of  five  hours,  and  then  ceased  entirely.  The 
least  contact  with  the  conductor  would  have  occasioned 
instant  death,  the  stream  of  fluid  far  exceeding  any- 
thing I  have  ever  witnessed,  excepting  during  a  thun- 
der-storm." 


What  instance  is  given  of  an  electric  fog  1 


156  ELECTRICAL    PHENOMENA. 

SPONTANEOUS  ELECTRICITY. 

332.  ST.  ELMO'S  FIRE.  When  in  a  darkened  room 
a  needle  is  brought  near  to  the  charged  conductor  of  an 
electrical  machine,  the  point  is  tipped  with  a  vivid  light, 
caused  •  by  the  flow  of  electricity  from  the  conductor  to 
the  needle.  In  the  same  manner  when  thunder-clouds 
approach  very  near  the  earth,  lightning  does  not  always 
occur;  but  the  electricity  becomes  so  intense,  that  it 
escapes  from  one  to  the  other  by  points  upon  the  surface 
of  the  earth,  which  then  glow  with  a  brilliant  flame. 
This  phenomenon  has  received  the  appellation  of  St. 
Elmo's  fire.  It  was  known  to  the  ancients  by  the  name 
of  Castor  and  Pollux,  and  many  instances  have  been  re- 
corded by  classic  writers.  On  the  night  before  the  bat- 
tle that  Posthumius  gained  over  the  Sabines,  the  Roman 
javelins  emitted  a  light. like  torches;  and  Caesar  relates 
that  during  the  African  war,  in  the  month  of  February, 
(here  suddenly  arose,  about  the  second  watch  of  the 
night,  a  dreadful  storm  that  threw  the  Roman  army 
into  great  confusion,  at  which  time  the  points  of  the 
darts  of  the  fifth  legion  appeared  to  be  onjire. 

383.  The  fire  of  St.  Elmo  is  often  finely  displayed 
upon  the  masts  of  vessels.  An  extraordinary  instance, 
which  happened  in  1696,  is  thus  related  by  Count  For- 
bin  :  "In  the  night  it  became  extremely  dark,  and  thun- 
dered and  lightened  fearfully.  We  saw  upon  different 
parts  of  the  ship  about  thirty  St.  Elmo's  fires  ;  among 
the  rest  was  one  upon  the  top  of  the  vane  of  the  main- 
mast, about  eighteen  inches  long.  I  ordered  one  of  the 
sailors  to  take  the  vane  down,  but  he  had  "scarcely  re- 
moved it  when  the  fire  again  appeared  upon  the  top  of 
the  mast,  where  it  remained  for  a  long  time,  and  then 
gradually  vanished."  When  Lord  Napier  was  on  the 
Mediterranean,  in  June,  1818,  he  observed,  during  a 
dark  and  stormy  night,  a  blaze  of  pale  light  upon  the 
mainmast  of  his  vessel.  It  appeared  near  the  summit, 


What  is  the  cause  of  spontaneous  electricity  ? 
What  name  has  been  given  to  this  phenomenon  ? 
Relate  the  several  instances. 


SPONTANEOUS    ELECTRICITY  157 

and  extended  about  three  feet  downward,  flitting  and 
creeping  around  the  surface  of  the  mast.  The  heads  of 
the  other  two  masts  presented  a  similar  appearance. 
At  the  end  of  half  an  hour,  the  flames  were  no  longer 
visible. 

384.  This  phenomenon  frequently  occurs  on  the  sum- 
mits of  mountains,  when  thunder  clouds  pass  near  them. 
Saussure  observed  it  upon  the  Alps,  in  1767.     On  ex- 
tending his  arm,  he  experienced   slight  electric  shocks, 
accompanied  by  a  whistling  sound,  and  obtained  distinct 
sparks  from  the  gold  button  of  a  hat  belonging  to  one  of 
his  party.     It  is  often  noticed  at  Edinburg  castle,  which 
stands  upon  a  high  rock,  250  feet  above  the  surrounding 
country.     Upon  the  approach  of  a  storm,  the  bayonets 
of  the  soldiers  mounting  guard   are  frequently  seen  cap- 
ped with  flame,  and  an  iron  ramrod,  placed  upright  upon 
the  walls,  presents  a  like  appearance. 

A  singular  instance  of  spontaneous  electricity  took 
place  at  Algiers,  on  the  8th  of  May,  1831.  During  the 
evening  of  this  day,  as  some  French  officers  were  walk- 
ing with  their  heads  uncovered,  each  was  surprised  at 
seeing  the  hairs  upon  the  heads  of  his  companions  erect, 
and  tipped  with  flame. '  Upon  raising  their  hands,  they 
perceived  a  similar  light  flitting  upon  the  ends  of  their 
fingers. 

A  remarkable  case  of  this  kind  was  observed  by  Pres. 
Totten,  of  Trinity  College,  at  Hartford,  Ct.,  in  the  month 
of  Dec.  1839.  As  this  gentleman  was  walking  one 
evening  in  the  midst  of  a  heavy  snow-storm,  protected 
by  an  umbrella,  his  attention  was  arrested  by  moment- 
ary flashes  of  light,  which  at  intervals  illumined  his 
path.  The  source  of  the  light  was  detected  upon  meet- 
ing another  person,  the  point  of  whose  umbrella  was 
seen  covered  with  flame,  which  was  constantly  escaping 
in  flashes.  The  light  first  noticed  by  Pres.  Totten,  pro- 
ceeded from  his  own  umbrella. 

385.  ELECTRIC  RAIN,  HAIL,  AND  SNOW.   Numerous 


When  does  it  occur  on  the  summits  cf  mountains  1 
State  the  cases  in  Art.  384. 


158  ELECTRICAL    PHENOMENA. 

and  well  attested  instances  have  occurred,  in  wbicn 
rain,  hail,  and  snow,  have  displayed  flashes  of  elec- 
tric light,  but  we  will  confine  ourselves  to  a  few.  On 
the  22d  of  Sept.  1773,  in  a  thunder-storm  which  fell 
upon  Skara,  in  Sweden,  the  drops  of  rain  were  seen  to 
strike  fire  and  sparkle  as  they  touched  the  ground. 

On  the  28th  of  Oct.  1772,  as  the  Abbe  Bertholon  was 
traveling  between  Brignai  and  Lyons,  in  the  midst  of  a 
heavy  storm,  he  was  surprised  at  seeing  the  rain-drops 
and  hail-stones  emitting  jets  of  light,  as  they  fell  upon 
the  metallic  parts  of  his  horse's  trappings. 

It  is  also  recorded,  that  the  miners  of  Freyburg,  on 
the  25th  of  January,  1822,  beheld  the  sleet  which  fell 
during  a  storm  Jlash  with  light  as  it  struck  the  earth. 

386.  It  is  not  difficult  to  explain  these  phenomena ; 
we  have  only  to  suppose,  that  the  electric  intensity  of 
the  atmosphere  and    the  earth  is  at  these  times  very 
great,  and  that  the  electricity  of  the  falling  bodies  is  the 
opposite  in  kind  to  that*  of  the  ground  and  of  the  objects 
upon  it.     At  the  moment  of  contact  the  two  kinds  of 
electricities  combine,  their  union  (as  is  always  the  case 
when  their  intensity  is  great)  being  indicated  by  a  sud- 
den flash. 

387.  ELECTRIC  ACTION  UPON  TELEGRAPHIC  WIRES. 
It  is  not  unusual  for  the  electricity  of  the  atmosphere 
to  exert  an  extraordinary  influence  upon  the  wires  of 
the  electrical   telegraph.      According   to   Prof.   Henry, 
this  influence  may  arise,  as  follows,  in  several  different 
ways. 

388.  First.     The  wire  may  be  struck  by  a  direct  dis- 
charge of  lightning  from  the  clouds.     An  instance  of 
this  kind  occurred  on  the  20th  of  May,  1846,  when  the 
lightning  struck  the  wire  of  the  telegraph,  at  the  place 
where  it  crosses  the  Hackensack  river.     From  the  point 

Relate  the  instances  given  of  electric  rain,  hail  and  snow. 
How  are  these  facts  explained  1 

What  are  Prof.  Henry's  views  respecting  the  influence  of  atmospheril 
electricity  upon  telegraphic  wire  1 
What  is  the  first  mode  of  action  1 
Give  the  instance. 


ACTION  UPON  TELEGRAPHIC  WIRES.      159 

where  the  discharge  took  place,  the  fluid  passed  along 
the  wire  each  way  for  a  distance  of  several  miles,  strik- 
ing- off  at  irregular  intervals  down  the  supporting  poles. 
Wherever  a  pole  was  struck,  a  number  of  sharp  explo- 
sions were  successively  heard,  like  the  rapid  reports  of 
several  rifles. 

389.  Secondly.    The  state  of  the  wire  may  be  disturb- 
ed by  the  conduction  of  a  current  of  electricity  from  one 
portion  of  space  to  another,  without  the  presence  of  a 
thunder-cloud ;  and  this  will  happen  in  the  case  of  a 
long  line,  when  the  electric  condition  of  the  atmosphere 
which  surrounds  the  wire  at  one  place  is  different  from 
that  at  another. 

390.  This  difference  in  the  electric  condition  of  the 
atmosphere  may  result  from  a  difference  in  elevation. 
(Art.  322.)    A  wire,  raised  by  means  of  a  kite,  gives  sparks 
of  positive  electricity,  in  a  perfectly  clear  day ;  hence,  if 
the  telegraphic  wires  pass  over  a  high  mountain-ridge,  a 
current  of  electricity  will  be  continually  flowing,  during 
serene  weather,  from   the  more  elevated  to  the  lower 
parts  of  the  wire. 

A  current  may  also  arise  in  a  long,  level  line,  if  a/o^ 
exists  at  one  end,  while  the  sky  is  clear  at  the  other ;  or 
if  a  storm  of  rain  or  snow  occurs  at  some  portion  of  the, 
line,  while  the  remainder  is  free  from  its  presence. 

Currents  of  electricity  have  been  produced  by  some 
of  these  causes,  of  sufficient  power  to  set  in  motion  the 
working  machine  of  the  telegraph.  In  one  case  it  began 
to  operate  spontaneously,  without  the  aid  of  the  battery, 
when  a  snow-storm  prevailed  at  one  end  of  the  line,  and 
clear  weather  at  the  other. 

391.  Thirdly.      The  inductive  action  of  a  thunder- 
cloud may  also  change  the  natural  electric  condition  of 
the  wire.     If,  for  example,  a  cloud  positively  electrified, 
is  moving  across  the  direction  of  the  wire,  it  will  con- 


What  the  second  1 

When  will  this  disturbance  happen  in  the  case  of  a  long  line  1 

How  may  this  difference  arise  7 

What  is  stated  respecting  the  energy  of  the  currents  thus  produced  1 

What  is  the  third  mode  of  influence  1 


160  ELECTRICAL    PHENOMENA. 

tinue,  as  it  gradually  approaches  the  line,  to  drive  ic 
the  remote  extremities  of  the  wire,  more  and  more  of 
the  positive  electricity  residing  in  it,  and  thus  occasion  a 
current.  As  the  cloud  gradually  recedes,  the  repulsion 
it  exerts  is  diminished,  and  a  current  then  arises  in  the 
opposite  direction. 

392.  Fourthly.  Every  flash  of  lightning  which  oc- 
curs within  many  'miles  of  the  line,  produces  poweiful 
electrical  currents  in  the  telegraphic  wires. 

To  this  influence  Prof.  Henry  attributes  the  phenom- 
ena witnessed  by  himself,  on  the  19th  of  June,  1816,  in 
the  telegraph  office,  at  Philadelphia,  and  which  he  thus 
describes.  "In  the  midst  of  the  hurry  of  the  transmis- 
sion of  the  congressional  intelligence  from  Washington 
to  Philadelphia,  and  thence  to  New  York,  the  apparatus 
began  to  work  irregularly.  The  operator  at  each  end 
cf  the  line  announced  at  the  same  time  a  storm  at 
Washington,  and  another  at  Jersey  City.  The  portion 
of  the  telegraphic  wire  which  entered  the  building,  and 
was  connected  with  one  pole  of  the  galvanic  battery, 
happened  to  pass  within  the  distance  of  less  than  an 
inch  of  the  wire,  which  served  to  form  the  connection 
of  the  other  pole  with  the  earth.  Across  this  space,  at 
intervals  of  every/ew?  minutes,  a  series  of  sparks  in  rapid 
succession  was  observed  to  pass  ;  and  when  one  of  the 
storms  arrived  so  near  Philadelphia  that  the  lightning 
could  be  seen,  each  series  of  sparks  was  found  to  he 
simultaneous  with  a  flash  in  the  heavens.  Now  we 
cannot  suppose,  for  a  moment,  that  the  wire  was  actu- 
ally struck  at  the  time  each  flash  took  place,  and  indeed 
it  was  observed  that  the  sparks  were  produced,  when 
the  cloud  andjlash  were  at  the  distance  of  several  ?nile* 
to  the  east  of  the  line  of  the  wire.  The  inevitable 
conclusion  is,  that  all  the  exhibition  of  electrical  phe- 
nomena witnessed  during  the  afternoon,  was  purely  the 
effect  of  induction,  or  the  mere  disturbance  of  the  natu- 
ral electricity  of  the  wire  at  a  distance,  without  any 
transfer  of  the  fluid  from  the  cloud  to  the  apparatus. 

What  the  fourth  1 

Describe  ihe  phenomena  witnessed  by  Professor  Henry. 


ACTION  UPON  TELEGRAPHIC  WIRES.     161 

"  The  discharge  between  the  two  portions  of  the  wire 
continued  for  more  than  an  hour,  when  the  effect  be- 
came so  powerful,  that  the  superintendent,  alarmed  for 
the  safety  of  the  building,  connected  tbe  long  wire  with 
the  city1  gas  pipes,  and  thus  transmitted  the  current 
silently  to  the  ground." 

393.  By  a  simple  apparatus,  Professor  Henry  rendered 
manifest  the  inductive  action  of  the  lightning-flash. 
The  arrangement  consisted  of  a  copper  wire,  fastened  at 
one  end  to  a  building,  and  extended  to  another,  400  feet 
distant.  Here  it  entered  the  Professor's  study,  and  thence 
passed  through  a  cellar  window  into  an  adjoining  well. 
With  every  flash  of  lightning  that  occurred  within  a  cir- 
cle of  twenty  miles,  needles  were  magnetized  in  the 
study  by.  the  induced  current  of  electricity  developed  in 
the  wire.  (C.  1033.) 

What  apparatus  was  constructed  by  this  gentleman  for  exhibiting  ;ti» 
inductive  action  of  tho  lightning  7 
What  effect  was  produced  1 


Wv 


PART    V. 

OPTICAL    PHENOMENA. 


CHAPTER  I. 

OF  THE  COLOR  OF  THE  ATMOSPHERE  AND  CLOUDS. 

394.  COLOR  OF  THE  ATMOSPHERE.     This  is  caused 
by  the   decomposition  of  the   solar   light.     It   is  well 
known,  from  the  experiment  of  the  prism  (C.  788),  that 
the  white  light  of  the  sun  consists  of  seven  colors  ;  and, 
that  of  all  these,  the  violet  and  blue  rays  have  the  least 
power  to  overcome  any  resistance  they  meet  with ;  and 
consequently  deviate  most  from  their  original  course  in 
passing  through  the  prism. 

395.  The  action  of  the  atmosphere  upon  the   sun- 
beams in  their  passage  to  the  earth,  is  precisely  similar 
to  that  of  a  prism. 

After  entering  the  atmosphere  they  are  constantly 
passing  in  their  onward  progress,  from  rarer  into  den- 
ser media,  and  are  therefore  decomposed.  A  portion 
of  the  blue  rays,  unable  to  overcome  the  resistance  of 
the  air  are  scattered  throughout  its  extent ;  and  being 
reflected  from  its  particles,  tinge  the  sky  with  an  azure 
hue ;  for  it  is  to  be  remarked,  that  a  body  appears  of 
the  same  color  as  the  light  it  reflects  and  by  which  it  is 
seen. 

396.  CYANOMETER.    In  order  to  determine  the  inten- 
sity of  the  blue,  a  cijanometer  is  employed,  an  instru- 

What  is  the  subject  of  part  fifth  1 

Of  what  does  chapter  first  treat  ? 

What  is  the  cause  of  the  color  of  the  atmosphere  ? 

Foi  what  purpose  is  the  cyanomcter  employed  ? 


COLOR    OF    THE    ATMOSPHERE.  163 

ment  which  derives  its  name  from  the  Greek  words, 
kuanos,  azure,  and  metron,  measure.  That  of  Saussure 
is  made  in  the  following  manner : 

A  circular  card  is  divided  into  fifty-one  parts,  and  each 
is  painted  of  a  different  shade  of  blue  increasing  from 
the  palest  tint,  formed  by  a  union  of  blue  and  white,  to 
the  deepest  produced  by  a  mixture  of  blue  and  black. 
The  colored  card  being  held  in  the  hand,  the  observer 
marks  the  particular  tint  corresponding  to  the  color  of 
the  sky,  and  its  number,  counting  from  the  palest  shade, 
denotes  the  intensity  of  the  azure. 

397.  EFFECT  OF  LATITUDE.     The  brilliancy  of  the 
sky  decreases  with  the  latitude.     Humboldt  discovered 
that  at  corresponding  heights  above  the  horizon,  the  blue 
in  19°  N.  Lat.  was  two  shades  below  that  in  16°  N.  Lat. 
The  intensity  also  at  Cumana,  10°  N.  Lat.,  is  twenty- 
four,  while  the  average  tint  for  Europe  is  only  fourteen. 

398.  This  diminution  in  brilliancy  is  caused  by  the 
less  perfect  absorption  of  the  atmospheric  .humidity  in 
the  temperate  and  arctic  regions,  than  in  the  equatorial 
climes — a  circumstance  arising  from   their  comparative 
low  mean  temperature,  and  consequent  decrease  in  the 
capacity  of  the  air  for  moisture. 

399.  In  the  same  place,  the  color  increases  in  bright- 
ness from  the  horizon  to  the  zenith — the  point  in  the 
heavens  directly  over-head.     Baron  Humboldt  found  in 
16°  N.  Lat.,  that  his  cyanometer  indicated  the  3d  shade 
at  the  horizon,  but  at  an  altitude  of  60°,  or  two-thirds  of 
the  distance  to  the  zenith,  the  22d  tint.    The  blue  of  the 
sky  is   palest  at  the  horizon,  in  consequence  of  being 
mixed  with  and  diluted  by  the  thin  vapors  of  the  air, 
which  settle  down  towards  the  earth. 

Within  the  torrid  zone,  the  sky  undimmed  by  vapors 
glows  with  the  purest  azure  ;  and,  under  like  circum- 
stances, in  regions  beyond  the  tropics,  the  same  bright 


Describe  this  instrument. 

How  is  the  brilliancy  of  the  sky  affected  by  latitude  ? 

State  the  result  of  observations.     How  is  this  difference  accounted  lor? 

What  is  the  law  in  respect  to  altitude  at  the  same  place  7 

Give  the  observations.    What  is  said  of  the  torrid  zone  ? 


164  OPTICAL    PHENOMENA. 

sines  are  seen.  Over  Italy.  California  and  the  Canary 
Isles,  hangs  a  canopy  of  the  deepest  blue,  and  even  on 
the  western  coast  of  Spitzbergen,  the  rich  azure  of  the 
heavens  has  equaled  at  times  the  splendid  hue  of  the 
tropic  skies. 

400.  EFFECT  OF  ALTITUDE.     In  ascending  from  the 
plains  to  the  mountains,  the  vapors  are  left  below,  the 
purity  of  the  atmosphere  increases,  and  the  pale  tint  of 
the  sky  changes  to  a  vivid  blue.    This  fact,  long  known 
to  the  chamois  hunters  of  Switzerland,  was  verified  by 
the  observations  of  Saussure  upon  the  Alps,  and  those 
of  Humboldt  on  the  Cordilleras. 

401.  Capt.  Mundy  thus  speaks  of  the  color  and  pure- 
ness  of  the  air  at  Simla,  which   is  the  most  northern 
European  settlement  in  India,  and  possesses  an  altitude 
of  7,800  feet.     "  To  the  north  of  Simla,  the  mountains 
rise  gradually  one  above  another,  until  the  panorama  is 
majestically  terminated  by  the  snowy  crescent  of  the 
great  Himalaya  belt,  fading,  on  either  hand,  into  indis- 
tinct distance.     In  fine  weather,  these  stupendous   icy 
peaks  cut  the  dark   blue  sky  with  such  sharp  distinct- 
ness of  outline,  that  their  real  distance  of  sixty  or  seven- 
ty miles  is,  to  the  eye  of  the  gazer,  diminished  to  one- 
tenth  part." 

402.  Brantz   Mayer,  in  his-  interesting  work   upon 
Mexico,  thus  alludes  to  the  same  facts. 

"  The  moonlight  of  Mexico  is  marvelously  beautiful. 
That  city  is  7,500  feet  above  the  level  of  the  sea.  The 
light  comes  pure  and  pellucid  from  heaven.  You  seem 
able  to  touch  the  stars,  so" brilliantly  near  do  they  stand 
out,  relieved  against  the  back-ground  of  an  intensely 
blue  sky.  Strolling  on  such  a  night  in  Mexico,  I  saw  the 
sharp  lines  of  tower  and  temple  come  boldly  out  with 
shape  and  even  color  almost  as  bright  as,  yet  softer  than 
at  noon-day." 

403.  At  Mussoori,  a  village  situated  upon  the  first 

What  is  said  of  Italy,  California,  the  Canary  Isles  and  Spitzbergen? 
What  is  the  law  in  regard  to  altitude  at  different  stations? 
What  is  said  of  the  blueness  of  the  sky  and  the  purity  of  the  atmosphere 
on  the  Alps  and  the  Cordilleras,  on  the  Himalayas  and  in  Mexico  1 


COLORS    OF    CLOUD3.  165 

range  of  the  Himalayas,  7,500  feet  above  the  sea,  so 
remarkably  clear  is  the  air  in  the  month  of  November, 
according  to  Lieut.  Bacon,  that  the  white  houses  at 
Moozaffirnuggur,  a  distance  of  eighty-two  miles,  have 
been  distinctly  seen  with  the  aid  of  a  spy-glass. 

404.  When,  however,  very  lofty  elevations   are  at- 
tained, the  heavens  assume  a  blackish  hue  ;  for  a  great 
portion  of  the  atmosphere  is  then  beneath  the  observer, 
and  but  little  blue  light  is  reflected  from  the  compara- 
tively small  number  of  particles  composing  the  attenu- 
ated air  above.     The  celestial  orbs  there  shine  with  a 
singular  brilliancy,  since  their  light  reaches  the  eye  be- 
fore its  lustre  has  been  dimmed,  in  consequence  of  pass- 
ing through  the  dense  strata  of  the  atmosphere  near  the 
surface  of  the  earth. 

405.  Captain  HodgsTm  remarked,  near  the  sources  of 
the  Ganges,  that  the  tint  of  the  sky  was  a  dark  blue, 
approaching  to  blackness;  and  that  the  stars  in  their 
rising  emerged  with  a  sudden  flash  from  behind   the 
snowy  peaks  of  the  Himalayas.     At  Zinchin,  sixteen 
thousand  feet  above. the  sea  level,  the  heavens  appeared 
of  a  dark  black  color,  the  sun  shining  without  the  least 
haze.     At  night,  that  part  of  the  horizon,  where  the 
moon  was   expected   to  rise,  could  scarcely  be  distin- 
guished by  the  irradiation  of  her  beams  before  the  orb 
touched  it,  and  the  stars  and  planets  shone  with  a  daz- 
zling light. 

406.  COLORS  OF  CLOUDS.     These  are  attributed  to 
the  power  which  the  atmosphere  possesses  of  absorbing 
light  (C.802),  in  common  with  other  transparent  media. 
When  a  sun-beam  falls  upon  the  ocean,  the  more  refran- 
gible rays  are  successively  absorbed  as  the  light  contin- 
ues to  pierce  the  translucent  water;  until,  at  last,  far 
beneath  the  surface,  nothing  but  red  light  is  perceived, 
according  to  the  statements  of  divers,  and  of  those  who 


What  is  the  color  of  the  sky  when  very  lofty  heights  are  attained  1 
Relate  the  remarks  of  Capt.  Hodgson. 

Explain  the  cause  of  the  colors  of  clouds,  and  the  manner  in  which  they 
ariee. 


166  OPTICAL    PHENOMENA. 

have  been  engaged  in  submarine  researches.     The  ac- 
tion of  the  atmosphere  is  precisely  the  same. 

In  the  morning'  and  evening,  the  sun-light  traverses 
the  densest  portions  of  the  air,  and  passes  through  a 
onger  track  than  at  any  other  time.  So  much  thicker 
and  more  dense  is  the  stratum  of  air  upon  the  horizon 
than  the  stratum  over-head,  that  the  sun-light  is  dimin- 
ished 1300  times  in  traversing  the  former ;  and  of 
10.000  rays  falling  towards  the  surface  of  the  earth, 
8,123  arrive  at  a  given  point  if  they  pass  perpendicu- 
larly through  the  air,  but  only  five  if  they  come  through 
a  horizontal  stratum.  From  these  causes,  the  more  re- 
frangible rays,  and  especially  the  violet  and  blue,  are  un- 
able to  struggle  through  and  are  absorbed  ;  while  the 
rest  emerge,  and  being  reflected,  from  the  light  masses 
of  vapor  floating  in  the  sky,  clothe  them  with  their  own 
bright  hues. 

407.  The  three  most  powerful  rays  of  the  solar  spec- 
trum are  red,  orange,  and  yellow  ;  and  these  colors  are 
the  common  tints  assumed  by  clouds.     At  times,  how- 
ever, they  glow  with  the  richest  variety  of  hues,  partic- 
ularly beneath  the  tropic  skies.    In  those  regions,  green, 
violet  and  purple  clouds  are  not  of  unfrequent  occurrence. 

Bishop  Heber,  on  his  passage  to  India,  beheld,  one 
evening  at  sunset,  when  near  the  equator,  large  tracts 
of  cloud  of  a  pale,  translucent  green,  surpassing  in 
beauty  every  effect  of  paint,  glass,  or  gem. 

The  sunsets  of  California  are  among  the  most  beauti- 
ful in  the  world,  and  the  clouds  that  rise  from  the  Pacific 
are  bathed  in  exquisite  tints  of  green,  purple,  and  violet. 

408.  Clouds,   possessing    these   singular   colors,   are 
rarely  seen  in  the  higher  latitudes  ;  they  are  however 
not   entirely  unknown.     Violet  clouds  have  been  wit- 
nessed at  Avignon,  in  France,  and  also  in  a  most  gorge- 
ous sunset  that  occurred  at  Hartford,  Ct.,  on  the  3d  of  Ju- 
ly, 1844,  and  which  presented  the  following  phenomena. 

409.  The  day  had    been  showery,  but  towards    its 


What  is  said  as  to  the  diminution  of  light  1 

In  what  regions  are  the  richest  tints  beheld  1    Give  instances. 

Where  are  violet  and  green  clouds  most  frequently  seen  1 


COLORS    OF    CLOUDS.  1(V 

close,  the  dense  canopy  of  clouds  was  broken  up,  and 
the  eastern  sky  filled  with  light  and  floating  masses 
of  vapor.  Soon  after  sunset,  the  stratum  of  clouds 
which  rested  upon  the  western  horizon,  rose  throughout 
its  whole  length,  revealing  between  the  mountains  and 
its  lower  edge  a  belt  of  sky  of  the  purest  azure.  Above 
this,  the  whole  field  of  vapor  was  gleaming  with  a  rich 
amber  light,  which,  as  it  streamed  through  rarer  or 
denser  portions  of  the  mass,  presented  every  phase  of 
brilliancy  and  depth  ;  at  the  same  time  displaying  the 
curiously  wrought  structure  of  the  airy  fabric.  When 
the  rays  of  the  sun  fell  upon  the  fragments  of  vapor 
floating  in  the  eastern  quarter  of  the  heavens,  their  jut- 
ting heads  and  broken  edges  gleamed  with  a  flame-like 
hue ;  while,  between  the  masses,  the  sky  appeared  of 
the  deepest  indigo.  As  the  evening  advanced,  portions 
of  the  western  stratum  assumed  the  tints  of  lead,  lake, 
pink,  green,  purple,  violet,  orange  and  crimson.  About 
eight  o'clock,  the  vapor  in  the  south-west  presented  a 
singularly  beautiful  appearance  ;  the  heavens  seemed 
as  if  covered  with  a  delicate  lace-work  woven  of  pris- 
matic rays,  and  this  phenomenon  was  succeeded  by  green, 
purple,  and  violet  clouds  in  the  west.  The  last  hue  of 
this  brilliant  pageant  was  an  intensely  vivid  crimson. 
which  was  gradually  lost  in  the  shades  of  night. 

410.  At  the  same  city,  in  May,  1845,  a  cloud  of  un- 
common  beauty  was   seen    by  the   writer   resembling 
marble  paper,  the  intermingling  colors   consisting  of 
bronze,  orange,  a  vivid  grass  green,  and  a  golden  yellow. 

411.  Green  clouds  occur,  when  the  vapor  is  illumin- 
ated at  the  same  time  by  the  deep  blue  light,  reflected 
from  a  distant  quarter  of  the  sky,  and  the  yellow  rays 
of  the  sun;  green  being  produced  by  the  union  of  blue 
and  yellow.     In  the  same  manner,  purple  and  violet 
clouds  appear,  when  they  glow  at  once  with  the  red 
rays  of  the  sun  and  the  azure  tint  reflected  from  the 
heavens;  since  purple  and  violet  arise  from  a  mixture 
of  red  and  blue  in  due  proportions. 

Describe  the  sunset  at  Hartford. 

Explain  the  origin  of  green  and  violet  clouds. 


168  OPTICAL    PHENOMENA. 

CHAPTER  II. 

OF  THE   RAINBOW. 

412.  The  rainbow  .is  that .  beautifully  colored  arch 
which,  at  times,  is  seen  during  a  shower  and  in  the  re- 
gion of  the  sky  opposite  to  that  where  the  sun  is  shin- 
ing. 

413.  When  perfect,  the  rainbow  consists  of  two  arches, 
the  inner,  called  the  primary  bow,  and  the  outer  the 
secondary,  each  composed  of  seven  colored  bows,  formed 
of  the   prismatic  hues,  viz.,  violet,  indigo,  blue,  green, 
yellow,  orange,  and  red.     Tn  the  primary  bow  the  red 
ring  occupies  the  highest  place,  the  orange  comes  next, 
and  so  on,  the  violet  assuming  the  lowest  position:  but 
in  the  secondary  the  order  of  colors  is  reversed. 

414.  The  cause  of  the  rainbow,  with  the  exception  of 
its  colors,  was  first  unfolded  by  Descartes ;  but  the  dis- 
covery by  Newton,  of  the  different  refrangibility  of  the 
sun's  rays,  enabled   this  great  philosopher  to  explain, 
with  the  utmost  completeness,  all  the  laws  of  this  brill- 
iant phenomenon. 

415.  In  order  to  understand  the  theory  of  the  rain- 
bow, we  must  have  recourse  to  diagrams. 

Imagine,  in  the  first  place,  that 
P  Q.  L,  figure  20.,  is  a  section  of 
a  globe  of  water,  and  that  S  P 
is  a  ray  of  light,  which,  passing 
through  the   hole  of  a  window- 
shutter  in  a  darkened  room,  falls 
upon  the  surface  of  the  globe  at 
P.     Here'  a  portion  of  the  light  is  ______^__ 

reflected,  and   the   rest,  entering  SECTION  OF  A  GLOBE  < 

the  globe  is  refracted  (C.  702)  and  decomposed  into  the 

seven  primary  colors  ;  one  of  which,  the  red  ray,  we  will 

What  is  the  subject  of  chapter  second  ? 

Define  the  rainbow. 

Of  what  does  it  consist  when  perfect  1 

Who  first  explained  the  cause  of  the  rainbow  1 

Trace  the  course  of  a  ray  of  light  in  figure  20. 


RAINBOW.  169 

now  alone  consider.  This  ray,  traversing  the  water, 
strikes  the  interior  surface  of  the  globe  at  Q,;  where  a 
part  of  its  light  is  lost  by  transmission,  and  escapes  into 
the  air, "while  the  remainder  is  reflected  to  the  point  L  ; 
here  the  light  is  once  more  subdivided,  one  portion  being 
refracted  to  the  eye  situated  in  the  direction  L  M,  and 
the  other  reflected  into  the  globe. 

That  the  results  are  such  as  have  been  described 
may  be  ascertained  by  placing  the  eye  at  the  point  Q,  of 
the  globe,  and  observing  likewise  the  course  of  the  ray 
through  the  air  and  water. 

416.  These  successive  transmissions  and  reflections 
are  unlimited  in  number,  and,  since  light  is  lost  at  each 
impact,  the  intensity  of  the  ray,  after  a  few  reflections, 
will  become  so  much  diminished,  that,  upon  its  emer- 
gence from  the  globe,  it  ceases  to  make  arty  impression 
upon  the  eye. 

417.  In  order  to  apply  these  remarks  to  the  subject 
before  us,  we  have  only  to  suppose,  that  figure  20.  is  the 
section  of  a  rain-drop,  instead  of  the  section  of  a  globe 
of  water ;  for  all  the  changes  which  light  undergoes  in 
one  case,  will  take  place  in  the  other. 

Thus  the  ray  of  light  S  P,  falling  upon  the  drop  at 
P,  is  refracted  towards  the  perpendicular  P  D,  reflected 
at  Q,,  and  refracted  from  the  perpendicular  L  D  as  it 
passes  into  the  air  at  L.  to  meet  the  eye  in  the  direction 
L  M :  all  these  results  occurring,  in  accordance  with 
well-known  optical  laws.  (C.  706,  709.) 

418.  In  the  case  supposed,  the  ray  S  P  suffers  two 
refractions  and  one  reflection,  and  if  it  strikes  the  drop 
more  or  less  obliquely,  different  quantities  of  light  will 
be  brought  to  the  eye  at  M.     Now  it  is  only  when  the 
greatest  amount  is  conveyed  to  the  eye  that  the  light  is 
sufficiently  intense  to  produce  any  impression  upon  the 
eight,  and   this  is  found  to  occur,  in  respect  to  the  red 


Apply  the  illustration  to  the  subject. 

In  the  case  supposed,  how  many  refractions  and  reflections  docs  the  ray 
undergo  1 

Under  what  circumstances  only  does  the  emergent  ray  make  any  im- 
pression upon  the  sight  ? 

8 


170  OPTICAL    PHENOMENA. 

ray,  when  the  angle  S  B  M,  figure  21.,  made  by  pro- 
longing the  lines  of  the  incident  and  emergent  rays  S  P 
and  M  Q,  till  they  meet  at  B,  and  called  the  angle  of 
deviation,  is  equal  to  42°  2'.  * 

This  angle  of  greatest  intensity  varies  however  for  each 
prismatic  color,  being  40°  17'  in  the  case  of  the  violet, 
and  increasing,  for  each  hue,  from  the  violet  to  the  red. 

419.  PRIMARY 
Bow.  If  we  consi- 
der P  a  L  D,  figure 
21.,  to  be  a  section 
of  a  rain-drop ;  of 
all  the  rays  that  fall 
upon  it  from  any 
one  point  in  4»e  sun, 
some,  as  S  P,  will  so 
strike  it,  as  to  meet 
the  eye  of  the  observ- 
er (supposed  to  be  at  SBCTION  op  A  RAIN.DROP 

M)  With  thegrCdtest  One  Reflection-two  Refractions. 

*  An  angle  is  the  opening  between  two  straight  lines  that  meet  each  other. 

Thus  the  opening  between  the  straight 
lines,  A  B  and  C'  B.-which  meet  at  B,  is 
called  the  angle  B,  or  the  angle  ABC; 
the  letter  at  the  point  of  meeting  always 
being  placed  in  the  middle.  The  size  of 
an  angle  is  computed  as  follows.  The 
circumference  of  any  circle  being  divided 
into  360  equal  parts,  each  part  is  called  a 
degree ;  a  degree  being  divided  into  60 
equal  parts,  each  part  is  called  a  minute; 
and  a  minute  being  divided  into  60  equal 
parts,  each  part  is  called  a  second.  If 
now  we  take  B  as  the  centre  of  a  circle,  • 

and  describe  the  circumference,  G  E  F,  cutting  the  two  lines,  A  B  and  C  B, 
In  any  two  points,  as  E  and  P,  the  number  of  degrees,  minutes  and  seconds 
contained  in  the  part  of  the  circumference,  E  P,  included  between  the  two 
lines,  A  B  and  C  B,  gives  the  vnlue  of  the  angle,  A  B  C.  For  example,  if  the 
length  of  the  circumference,  G  E  P,  was  so  great  that  it  measured  360  inches, 
and  the  part,  E  F,  contained  40  inches  and  nine-sixtieths  of  an  inch,  ABC 
would  be  angle  of  forty  degrees  and  nine  minutes  (40°  9').  Dcgroi-s  ai<  desig- 
nated by  the  following  character,  °;  minutes  thu"s,  ';  and  seconds  thus.  ". 

What  is  the  angle  of  deviation  for  the  red  ray,  when  the  greatest  amount 
ut  light  comes  to  the  eye  1 

What  is  the  angle  of  greatest  intensity  for  the  violet  1 
Explain  figure  21. 


RAINBOW.  171 

possible  brilliancy  :  making  the  angle  S  B  M  equal  to 
42°  2',  in  the  case  of  red  light.  If  the  line  M  O  is  now 
drawn  parallel  to  S  B,  it  may  be  regarded  as  a  ray  of 
the  sun  passing  through  the  eye  of  the  spectator  ;  and 
since  all  the  rays  of  the  sun  are  parallel  to  each  other  at 
the  earth,  the  angle  B  M  O  will  also  be  equal  to  42°  2'. 

420.  The  observer  then  being  placed  with  his  back 
to  the  sun,  and  his  eye  at  M,  will  receive  the  impression 
of  red   light  from  the  drop  P  L  Q,  D,  in   the  direction 
B  M  ;  and  not  only  from  this  drop,  but  also  from  every 
other  drop,  whose  angular  distance  from  the  line  M  0 
is,  at  that  moment,  the  same. 

It  is  therefore  evident,  if  we  suppose  the  line  M  B  to 
turn  about  M  O,like  the  legs  of  a  pair  of  compasses,  that 
all  the  points  at  which  red  light  is  seen  lie  in  the  cir- 
cumference of  a  circle  whose  centre  is  O ;  and  that 
around  this  centre  an  arch  of  red  light  will  appear  in 
the  heavens. 

421.  The  breadth  of  this  arch  will  be  equal  to  the 
apparent  diameter  of  the  sun,  or  about  32 ;   for  what 
has  been  said  in  regard  to  rays  proceeding  from  any  one 
point  in  the  sun,  viz.,  that  some  of  them  will  reach  the 
eye  under  the  angle  of  greatest  brilliancy,  is  equally 
true  of  those  which  emanate  from  every  point  of  his  disk. 

422.  The  explanation  of  the  origin  of  the  red  arch  is 
equally  applicable  to  the  rest  of  the  colored  arches.    The 
latter  will  be  found,  however,  below  the  forme/-;    for, 
since  their  angles  of  greatest  brilliancy  are  each  less  than 
that  of  the  red,  they  must  consist  of  portions  of  smaller, 
concentric  circles. 

Thus,  the  violet  arch  can  only  be  seen  from  drops  below 
and  within  B,  when  the  light  that  meets  the  eye  coming 
in  the  direction  B2  M,  makes  the  angle  B2  M  O  equal 
to  40°  17'.  Between  the  violet  and  red  arches  the  other 
colored  bows  will  be  seen  arranged  in  the  order  of  the 
spectrum  ;  the  whole  forming,  by  their  union,  the  pri- 
mary bow. 

Explain  the  manner  in  which  the  red  arch  is  caused.  What  is  its  breadth  1 
Apply  the  same  mode  of  reasoning  to  the  other  colored  arches. 
How  is  the  primary  bow  formed  1 


IT2  OPTICAL    PHENOMENA. 

423.  SECONDARY  Bow.    The  secondary  bow  is  form' 
ed  when  the  sun's  rays,  entering  the  bottom  of  the  drop, 
suffer    two   reflections   from   the   interior   surface,   and 
emerging  at  the  top,  reach  the  eye  of  the  spectator  after 
two  refractions. 

The  course  of  the  ray  is  rig. 

seen  in  figure  22.,  where  S 
E  A  is  the  incident  ray, 
B  and  C  the  two  points  of 
reflection,  and  D  E  H  the 
emergent  ray,  supposed  to 
meet  the  eye  of  the  ob- 
server at  H. 

424.  So   much   light  is 
lost    by     these    successive 
changes    in  direction,  that 

only  at  certain  inclinations  SECTION  OP 

a  sufficient  quantity  reaches       TV*  Reflections-two  Refractions. 
the  eye,  from  each  of  the  prismatic  colors,  to  produce 
the  secondary  bow.     Its  tints,  after  all,  are  faint  com- 
pared with  those  of  the  primary. 

425.  The  violet  light  can   only  be  seen  when   the 
angle  of  deviation  S  E  H  is  54°  9',  and  the  red  when  it 
is  50°  59'.      Suppose,   as  in  the  case  of  the  primary 
bow,  that  H  L  is  the  direction  of  a  ray  from  the  sun 
passing  through  the  eye  of  the  observer,  and  making 
with  HE  the  angle  L  H  E  equal  to  the  angle  of  devia- 
tion.    If  then,  the  line  H  E  revolves  about  H  L,  the 
spectator,  with  his  back  to  the  sun  and  his  eye  at  H, 
will  behold  in  the  heavens,  between  the  limits  of  54° 
9'  and  50°  59'  a  prismatic  bow  consisting  of  similar  por- 
tions of  seven  concentric  circles  /  the  violet  arch  assum- 
ing the  highest  position  and  the  red  the  lowest.     The 
other  colors  occupy  intermediate    places ;    the   greater 
their  refrangibility  the  greater  their  elevation. 

Under  what  circumstances  docs  the  secondary  bow  occur  7 
Trace  the  course  of  the  ray  in  figure  22. 
What  is  said  of  the  brilliancy  of  the  secondary  bow  7 
What  must  be  the  size  of  the  angle  of  deviation  that  the  violet  light  caa 
«>e  seen  ?    What  the  size  that  the  red  ray  may  be  visible  1 
How  is  the  secondary  bow  formed  1 


RAINBOW. 


173 


Fig.  23. 


426.  The  subject  is 
further  illustrated  by 
the  following  figure, 
where  the  four  parallel 
lines  represent  rays  of 
the  sun  falling  upon 
four  drops  of  water,  and 
O  P  the  direction  of  an- 
other ray  imagined  to 
pass  through  the  eye  of 
the  spectator,  R  O  and 
V  O  are  the  red  and  vio- 
let rays  of  the  primary 
bow;  R  O  and  V  O 
the  red  and  violet  rays 
of  the  secondary  ;  and 
the  positions  of  the  red 
and  violet  arches  of  the 
two  bows  are  indicated 

by  the  dotted  lines.  The  other  colored  arches  are 
found  between  the  red  and  violet,  following  the  order  of 
colors  in  the  prismatic  spectrum.  P  is  the  centre  of  the 
rainbow. 

427.  In  the  explanation  just  given,  we  have  reasoned 
as  if  the  rain-drops  were  stationary,  which  of  course  is 
not  the  case  ;    but  this  supposition  leads  to  no  error, 
inasmuch  as  the  air  is  filled  with  rain-drops  during  the 
prevalence  of  a  shower,  and  before  one  set  of  drops,  by 
sinking  too  low,  ceases  to  present  to  the  eye  the  colors 
of  the  bow,  another  set  has  descended,  taken  their  place, 
and  is  performing  their  office.     While  the  observer  is 
stationary  the   rainbow  is  fixed  in  position,  but  the 
drops  that  give  rise  to  its  glowing  tints  are  continually 
changing: 

428.  BREADTH  OF  THE  Bows.    The  angular  distance 
from  the  middle  of  the  red  arch  to  the  middle  of  the 
violet  in  the  inner  bow,  is  the  difference  between  42° 


Illustrate  farther  from  figure  23. 

Why  is  the  bow  stationary  although  the  drops  are  in  motion  ? 


174  OPTICAL    PHENOMENA. 

2'  and  40°  17'  or  1°  45' ;  a  quantity  nearly  equal  to 
three  and  a  half  times  the  apparent  breadth  of  the  sun. 
This  space  is  occupied  by  the  remaining  five  colored 
arches,  and,  as  each  is  32'  in  width,  (Art.  421,)  they  ne- 
cessarily overlap  one  another,  and  cause,  by  their  mutual 
blending,  an  indistinctness  in  the  boundary  of  the  several 
hues.  The  two  half-breadths  of  the  red  and  violet 
arches  added  to  1  °  45'  give  the  whole  width  of  the  bow, 
which  is  equal  to  2°  17',  or  about  four  and  a  half  times 
the  apparent  diameter  of  the  sun.  • 

429.  The  breadth  of  the  exterior  bow,  from  the  mid- 
dle of  the  red  to  that  of  the  violet,  is  found  in  like  man- 
ner to  be  3°  10'— the  difference  between  54°  9'  and  50° 
59'.     To  this  quantity  32'  must  be  added  to  obtain  the 
entire  breadth. 

The  interval  between  the  bows,  computing  from  the 
red  of  the  primary  to  that  of  the  secondary,  is  8°  57'. 
All  these  results,  deduced  theoretically,  precisely  agree 
with  those  obtained  by  actual  measurement. 

430.  POSITION  AND  SIZE  OF  THE  RAINBOW.     Since 
the  centre  of  the  rainbow  is  in  the  direction  of  the  line 
imagined  to  be  drawn  from  the  sun  through  the  eye  of 
the  spectator,  its  position  will  evidently  vary  with  that 
of  the  spectator,  and  its  size  with  the  altitude  of  the  sun. 
If  this  luminary  is  42°  2'  above  the  horizon,  the  top  of 
the  inner  bow  will  be  just  visible  ;  but  if  upon  the  hori- 
zon, the  bow  will  be  a  semicircle,  having  an  elevation 
of  42°  2'.  If  the  observer,  in  the  latter  case,  were  upon 
the  summit  of  a  mountain,  the  arch  Avould  be  somewhat 
greater  than  a  semicircle  /  since  the  line  of  direction 
from  the  sun   through  his  eye,  would  strike   the  sky 
opposite,  at  a  point  above  the  horizon. 

Should  a  person  happen  to  be  upon  a  mountain, 
\vhen  the  sun  is  high  in  the  heavens,  and  a  shower  at 
the  same  time  occur  in  the  vale  below,  he  will  some- 
times perceive  a  rainbow  forming  a  complete  circle. 


State  what  is  said  in  regard  to  the  breadth  of  the  bows. 
What  in  respect  to  the  position  and  size  of  the  rainbow. 
When  are  entire  circles  beheld  ? 


EXTRAORDINARY    BOWS.  175 

Such  are  said  by  Ulloa  to  be  frequently  seen  on  the 
*  mountains  of  Peru  above  Quito. 

The  foaming  waters  of  cataracts  are  often  spanned 
by  richly  tinted  bows,  caused  by  the  rising  spray.  They 
are  regularly  seen  at  the^  falls  of  Schaffhausen,  on  the 
Rhine,  and  at  the  cataract  of  Niagara.  At  Terni,  in 
Italy,  where  the  river  Veiino  rushes  over  a  precipice  200 
feet  high,  a  bow  of  rare  beauty  is  beheld.  It  appears,  to 
a  spectator  below,  arching  the  falls  with  its  glowing 
tints,  while  two  other  bows  are  reflected  on  the  right. 
and  left. 

431.  RAINBOWS    IN   THE    NORTH.      Rainbows   are 
sometimes  seen  at  mid-day.     On  the  13th  of  Dec.  1847, 
at  one  o'clock,  P.  M.,  Prof.  Olmstead  beheld  at  Yale 
College  an  entire  bow  in  the  north.     During  the  same 
week,  the  writer  observed  at  Hartford  a  similar  bow 
at  nearly  the  same  hour  of  the  day.     Such  a  phenom- 
enon can  never  arise,  in  the  case  of  the  primary  bow, 
unless  the  sun's  altitude  at  the  time  is  considerably  less 
than  42°,  which  only  happens  in  the  winter. 

432.  EXTRAORDI- 
NARY Bows.    When 
the  light  of  the  sun  is 
reflected  from  the  sur- 
face of  tranquil  wa- 
ter, rainbows  of  sin- 
gular   form     are    at 
times  observed.     On 

the   6th   of    August,    ^A  :  CM! 

1(598,  Dr.  Halley  be- 
held,  while    walking 

on  the  walls  of  Chester,  by  the  river  Dee,  a  rainbow  of 
the  form  represented  in  figure  24.,  where  A  B  C  is  the 
primary  bow,  DBF  the  secondary,  and  A  H  G  C  the 
extraordinary  bow,  cutting  the  secondary  at  H  and  G. 
Its  colors  were  arranged  like  those  of  the  primary. 


Give  the  instances  of  rainbows  over  cataracts. 

When  can  rainbows  appear  in  the  north  7 

Explain  from  figure  24.  the  extraordinary  bow  seen  by  Halley. 


176  OPTICAL    PHENOMENA. 

433.  The  sun  was  shining  c.early  upon  the  calm  sur- 
face of  the  river,  and  Dr.   Halley  discovered  that  the* 
extraordinary  bow  was  nothing  more  than   the  rest  of 
the  circle  of  which  the  primary  was  apart,  bent  upwards 
by  reflection  from  the  water. 

A  similar  rainbow,  formed  by  reflection  from  the  river 
Eure,  was  beheld  at  Chartres,  in  1665  ;  when  a  faint 
arch  was  seen  crossing  the  primary  at  its  summit. 

434.  SUPERNUMERARY  Bows.     Arches  of  prismatic 
colors  are  sometimes  seen,  both  within  the  primary,  and 
without   the  secondary  bows,  to  which    the   name  of 
supernumerary  or  supplementary  bows  is  given. 

435.  On  the  5th  of  July,  1828,  Dr.  Brewster  saw  three 
supernumerary  bows  within  the  primary,  each  composed 
of  green  and  red  arches.      Outside  of  the  secondary  a 
red  arch  was  clearly  seen,  and  beyond  this  a  faint  green 
one. 

At  Montreal,  in  September,  1823,  three  supplementary 
bows  were  noticed  by  Prof.  Twining,  within  the  prima- 
ry ;  exhibiting  however,  only  a  single  color,  which  was 
violet  or  rather  a  dull  red. 

At  Hartford,  Cu,  on  the  5th  of  August,  1847,  at  sun- 
set, two  supernumerary  bows  were  seen  by  the  writer, 
within  the  primary,  extending  throughout  the  whole 
semicircle.  The  first,  in  contact  with  the  primary,  con- 
sisted of  green  and  red  arches,  and  the  second  of  a  sin- 
gle band  of  pale  red  light. 

The  most  remarkable  phenomenon  of  this  kind,  was 
that  observed  by  the  Rev.  Mr.  Fisher,  in  Dumfrieshire, 
and  related  by  Dr.  Brewster,  at  a  meeting  of  the  Brit- 
ish Association,  in  1840.  In  this  case  the  primary  was 
attended  \>y  five  supernumerary  bows,  and  the  secondary 
by  three.  Kaemtz  remarks,  that  it  is  not  easy  to  account 
for  these  supplementary  bows  in  a  satisfactory  manner; 
hut  according  to  Young,  Arago,  and  others  they  arise 
A-om  the  action  of  the  rays  of  light  upon  each  other :  the 
txplauation  however,  is  too  abstruse  to  be  here  introduced. 

436.  LUNAR  Bows.     Rainbows  are  sometimes  pro- 
Relate  the  cases  given  of  supernumerary  bows. 


MIRAGE.  177 

duced  by  the  light  of  the  moon ;  their  occurrence,  how- 
ever, is  extremely  rare,  and  their  tints  so  very  faint  as 
to  be  scarcely  perceptible.  One  of  the  most  brilliant 
ever  beheld,  was  seen  by  Mr.  Tunstall,  at  Gretna 
Bridge,  in  Yorkshire,  on  the  night  of  the  18th  of  Octo- 
ber, 1782.  It  became  visible  about  nine  o'clock,  and 
continued,  with  varying  degrees  of  brightness,  till  past 
two.  At  first  it  appeared  as  a  distinct  bow  without  colors, 
but  afterwards  the  tints  were  very  conspicuous  and  vivid, 
preserving  the  same  order  as  in  the  solar  bow,  though 
paler ;  the  red,  violet,  and  green  being  the  brightest. 
At  twelve  o'clock  it  attained  its  greatest  splendor.  This 
phenomenon  occurred  three  days  before  the  moon  was 
full ;  during  its  continuance,  the  wind  was  very  high, 
and  a  drizzling  rain  fell  for  most  of  the  time. 

Another  bow  was  seen  by  the  same  observer,  on  the 
27th  of  February,  in  the  same  year.  The  colors  were 
tolerably  distinct,  but  the  orange  appeared  to  predom- 
inate. A  lunar  bow  with  colors,  was  also  noticed  near 
Chesterfield,  about  Christmas,  in  the  year  1710,  and  is 
described  by  Thoresby  in  the  Philosophical  Transac- 
tions. 


CHAPTER    III. 

OF   MIRAGE. 


437.  When  a  ray  of  light,  proceeding  from  any  ob- 
ject, passes  obliquely  out  of  one  medium  into  another 
of  a  different  density,  it  is  refracted,  or  bent  from  ita 
course,  (C.  704,)  and  when  it  reaches  the  eye,  the  object 
is  seen  in  the  direction  of  the  last  refracted  ray. 


Relate  the  several  instances  of  lunar  bows. 
What  is  the  subject  of  chapter  third  1 

In  what  direction  is  an  object  seen,  when  the  rays  that  come  from  it  U» 
the  eye  first  pass  through  media  of  different  densities  1 


178  OPTICAL    PHENOMENA. 

438.  Thus,    if   E  Fig.  25. 

represents   the   earth,  "" 

and  1-2,  2-3,  3-4,  dif- 
ferent strata  of  the 
atmosphere,  decreas- 
ing in  density  from  1 
to  4,  a  ray  of  light 
proceeding  from  the 
star  S,  and  meeting 
the  exterior  stratum  of 
the  atmosphere  at  4, 

Will      be     successively  ATMOSPHBWC  REFBACTI 

refracted  in  the  directions  4-3,  3-2,  and  2-1  ;  so  that  a 
spectator  at  1  will  not  see  the  star  S  in  its  real  position, 
but  in  the  direction  of  1-2  S'.  For  this  reason  all  celes- 
tial objects,  (unless  in  the  zenith,  where  there  is  no  re- 
fraction,) appear  above  their  true  position.  (C.  703.) 
Thus,  the  sun  and  moon,  for  instance,  at  their  apparent 
rising  and  setting  are  actually  below  the  horizon. 

439.  The  variations  in  the  density  of  the  atmosphere 
near  the  earth,  produced  by  local  changes  in  tempera- 
ture, occasion  a  similar  displacement  of  terrestrial  ob- 
jects ;  this  is  ordinarily  seen  in  the  slight  elevation  of 
coasts  and  ships,  when  viewed  across  the  sea,  and  is 
then  called  looming  j  but  to  the  more  extraordinary 
phenomenon  of  this  nature,  the  name  of  mirage  has  been 
given.     When  this  phenomenon  occurs,  images  of  ships 
erect  and  inverted  are  seen  in  the  air,  delightful  visions 
of  tranquil  lakes  and  verdant  fields  delude  the  fainting 
traveler  of  the  desert,  and  sometimes,  as  in  the  case  of 
Reggio,  a  noble  city  with  all  its  splendid  panorama  of 
towers  and  arches,  stately  palaces  and  terraced  heights, 
appears  like  a  fairy  scene  upon  the  slumbering  waters 
of  the  sea. 


Explain  the  effect  of  atmospheric  refraction  from  figure  25. 
What  is  looming  ? 
What  is  mirage? 


179 


440.  INSTANCES..  On  the  first  of 
August,  1798,  Dr.  Vince  observed, 
at  Ramsgate,  a  vessel  in  the  dis- 
tance, the  topmast  only  being  visi- 
ble above  the  horizon,  as  at  A,  fig. 
26.     Two  complete  images  of  the 
vessel  were  seen  at  the  same  time 
in  the  air,  the  one  at  C  erect,  and 
the    other   below   at   B   inverted: 
between  them  a  distinct  image  of 
the  sea  appeared  at  D  E.     The 
two  images  were  still  visible  when 
the   real  ship  had  passed  entirely 
out  of  sight. 

441.  Similar    phenomena   were 
noticed  by  Capt.  Scoresby  in  1820, 
while  navigating  the  arctic  seas. 
In  one  instance  he  beheld  from  the 
mast-head  eighteen  sail  of  ships,  at 
the  distance  of  twelve  miles  ;    one 

appeared  taller  than  its  actual  height,  another  shorter  ; 
and  above  several  of  the  rest,  inverted  images  were  seen. 
In  1822,  he  recognized  his  father's  ship,  the  Fame,  by 
an  inverted  image  of  the  vessel  in  the  air.  though  it  was 
subsequently  found  to  have  been  at  that  time  thirty 
tniles  distant,  and  seventeen  miles  beyond  the  horizon. 

442.  During  the  late  Exploring  Expedition,  a  singu- 
lar instance  of  mirage  was  seen  off  Terra  del  Fuego, 
from  the  decks  of  the  Vincennes  and  Peacock,  and  which 
is  thus  related.     "  On  the  17th  of  February,  1839,  we 
had  an  extraordinary  degree  of  mirage  or  refraction  of 
the  Peacock,  exhibiting  three  images,  two  of  which  were 
upright  and  one  inverted.    They  were  all  extremely  well 
defined.     The  temperature  on  deck  was  54°  Fah.,  that 
at  the  mast-head  62°  Fah.    A  vessel,  that  was  not  in  sight 
from  the  Vincennes'  deck,  became  visible,  and  the  land 
was  much  distorted,  both  vertically  and  horizontally. 


Relate  the  several  oases  of  mirage,  IT  440 — 445. 


ISO  OPTICAL    PHENOMENA. 

On  board  the  Peacock,  similar  appearances  were  observ- 
ed of  the  Vincennes  and  Porpoise.  There  was,  however, 
a  greater  difference  between  the  mast-head  temperature 
and  that  on  deck,  the  thermometer  standing  at  62°  Fah. 
at  the  mast-head,  while  on  the  deck  it  was  out  50°  Fah., 
being  a  difference  of  12°;  that  on  board  the  Vincennes 
differed  only  8°." 

443.  Simpson,  while  exploring  the  coasts  of  the  north 
polar  seas,  in  the  summer  of  1837,  beheld  a  remarkable 
display  of  the  mirage.     As  he  rowed  over  the  tranquil 
ocean,  he  seemed  to  be  traversing  a  valley  ;  the  waters 
apparently  rising  on  either  hand,  like  the  sides  of  a 
mountain,  and  the  huge  icebergs  upon  theii  surface  ap- 
pearing ready  to  topple  down  upon  him. 

444.  During  the  march  of  the  French  army  over  the 
sandy  plains  of  Egypt,  many  instances  of  the  mirage 
occurred.     The  villages,  situated  upon  small  eminences, 
were   successively  seen   like   so    many  islands    in    the 
midst  of  an  extensive  lake,  and  beneath  each  village 
appeared  its  inverted  image.     In  the  same  direction,  an 
image  of  the  blue  sky  was  seen,  clothing  the  sand  with 
its  own  bright  hues,  and   causing  the  wilderness  to  ap- 
pear like  a  rich  and   luxuriant,  country.     So  complete 
was  the  deception,  that  the  troops  hastened  forward  to 
refresh    themselves   amid   these   cool    retreats ;    but,  as 
they  advanced,  the  illusion  vanished,  only  to  re-appear 
at  the  villages  beyond. 

445.  This  phenomenon  is  so  common  on  the  deserts 
of  Asia  and  Africa,  that  the  Koran  calls  every  thing  de- 
ceitful by  the  word  serab,  which  signifies  mirage.    It  re- 
marks, for  example,  that  "the  actions  of  the  incredulous 
are  like  the  serab  of  the  plain ;  he  who  is  thirsty  takes 
it  for  water,  and  finds  it  to  be  nothing." 

446.  While  Baron  Humboldt  was  at  Cumana,  he  fre- 
quently saw  the  islands  of  Picuita  and  Boracha,  appa- 
rently hanging  in  the  air,  and  sometimes  with  inverted 
images ;    and  at  Mesa  de  Pavona,  cows   were  beheld 

Where  does  this  phenomenon  frequently  occur? 
What  instances  are  given  by  Humboldt  and  Tschudi  ? 


FATA    MORGANA.  181 

seemingly  suspended  in  the  air,  at  the  distance  of  2,132 
yards. 

When  Dr.  Tschudi  and  his  party  were  traversing  a 
deep  sandy  plain,  near  the  river  Pasamayo  in  Peru, 
they  beheld  the  figures  of  themselves,  riding  over  their 
own  heads,  magnified  to  gigantic  proportions. 

447.  FATA  MORGANA.     This  name  is  given  to  an 
extraordinary  optical  phenomenon,  which  has  been  often 
seen  in  the  straits  of  Messina,  between  the  island  of 
Sicily  and  the  Italian  coast.     It  has  been  described  by 
many  writers,  and,  though  known  for  centuries,  has  but 
lately  been   considered  as   the  effect  of  mirage.     The 
following  is  the  description  by  Antonio  Minasi,  which  is 
regarded  as  the  most  correct. 

"  When  the  rising  sun  shines  from  a  point,  whence  its 
incident  ray  forms  an  angle  of  about  45°  on  the  sea  of 
Reggio,  and  the  bright  surface  of  the  water  in  the  bay 
is  not  disturbed  either  by  the  wind  or  the  current,  a  spec- 
tator placed  on  an  eminence  in  the  city  of  Reggio,  with 
his  back  to  the  sun,  and  his  face  to  the  sea,  suddenly 
beholds  in  the  water  numberless  series  of  pilasters, 
arches,  castles  well  delineated,  regular  columns,  lofty 
towers,  superb  palaces,  with  balconies  and  windows,  ex- 
tended valleys  of  trees,  delightful  plains  with  herds  and 
flocks,  armies  of  men  on  foot  and  horseback,  all  passing 
rapidly  in  succession  along  the  surface  of  the  sea." 

In  a  peculiar  state  of  the  atmosphere,  when  its  dense 
vapors  extend  like  a  curtain  over  the  waters,  the  same 
objects  are  not  only  reflected  from  the  surface  of  the  sea, 
but  are  likewise  seen  in  the  air,  though  not  so  dislinct 
or  well  defined,  and  if  the  atmosphere  is  slightly  hazy, 
the  images  seen  upon  the  surface  of  the  water  are  vivid- 
ly colored  or  fringed  with  all  the  prismatic  hues. 

448.  But  a  most  extraordinary  instance  of  the  mirage 
occurred  at  Hastings,  on  the  coast  of  Sussex,  on  the 
26th  of  July,  1798.     The  cliffs  of  the  French  coast  are 

fifty  miles  distant  from  this  town,  and  in  the  usual  state 
of  the  atmosphere,  are  below  the  horizon  and  completely 


Describe  the  Fata  Morgana. 


182  OPTICAL    PHENOMENA. 

hid  from,  view ;  but  on  the  day  mentioned,  at  five 
o'clock  P.  M.,  they  were  seen  extending  to  the  right 
and  left  for  several  leagues,  and  apparently  only  a  few 
miles  off.  As  the  narrator,  Mr.  Latham,  walked  along 
the  shore,  the  sailors,  who  accompanied  him,  pointed  out 
and  named  the  different  places  on  the  opposite  coast, 
which  they  were  accustomed  to  visit.  By  the  aid  of  a 
telescope,  oinall  vessels  were  plainly  seen  at  anchor  in 
the  French  harbors,  and  the  buildings  on  the  heights 
beyond  were  distinctly  visible. 

The  Cape  of  Dungeness,  which  at  the  distance  of  16 
miles  from  Hastings,  extends  nearly  two  miles  into  the 
sea,  appeared  quite  close  to  the  town,  and  the  fishing 
boats,  that  were  sailing  at  the  time  between  the  two 
places,  were  magnified  to  a  high  degree.  This  curious 
phenomenon  continued  in  its  greatest  beauty  for  more 
than  three  hours.  The  day  was  extremely  hot,  without 
a  breath  of  wind. 

449.  A  remarkable  mirage  of  Dover  Castle,  was  seen 
by  Dr.  Vince  and  another  gentleman,  on  the  6th  day  of 
August,  1806,  at  Ramsgate. 

F.jg.  27. 


MIRAGE  —  DOVEB  CASTLE. 


The  summits,  v  xw  y,oi  the  four  turrets  of  the  castle, 
(fig.  27.,)  are  usually  seen  beyond  the  hill  A  B,  which 
lies  between  the  castle  and  Ramsgate  ;  but,  on  this  day 
not  only  the  turrets  were  visible,  but  the  whole  castle, 
m  nr  s,  appeared  as  if  it  were  on  the  side  of  the  hill 
next  to  Ramsgate. 

Relate  the  account  of  the  mirage  at  Hastings,  and  of  that  at  Ramsgate.  j 


ERECT    AND    INVERTED    IMAGES.  183 

Between  the  observers  and  the  shore,  from  which  the 
hill  rises,  there  was  about  six  miles  of  sea,  and  from 
thence  to  the  top  of  the  hill  the  distance  was  about  the 
same.  Their  own  height  above  the  water  was  nearly 
seventy  feet. 

450.  ORIGIN.     The  cause  of  mirage  has  been  par- 
tially stated;  but  the  subject  demands  a  more  complete 
explanation.    The  phenomena  may  be  divided  into  three 
classes,  viz. :  those  produced  by  refraction,  those  pro- 
duced by  refraction  and  reflection  conjointly,  and  those 
produced  by  reflection  only. 

451.  The  image  of  Dover  Castle  was  probably  pro- 
duced by  refraction,  simply  ;  for  the  atmosphere  gradual- 
ly increasing  in  density  from  the  lofty  heights  of  the 
castle  to  the  level  of  the  sea,  the  rays  of  light  proceeding 
from  the  edifice,  reached  the  eyes  of  the  spectators  in  a 
curved  line,  like  those  which  emanate  from  a  star,  (Art. 
438,)  and  the  whole  structure  therefore  appeared  to  the 
observers  above  its  true  position. 

452.  Phenomena,  like  those  observed  by  Scoresby, 
are  attributed  to  the  combined  influence  of  refraction 
and  reflection.     At  such  times,  the  stratum  of  air  in 
contact  with  the  sea  is  colder  than  that  immediately 
above  (Art.  442),  and  this  likewise  colder  than  the  next 
superior  stratum,  and  so  on.     Consequently,  to  a  certain 
extent,  the  density  of  the  atmosphere  decreases  with  the 
distance  from  the  ocean,  and,  under  these  circumstances3 
the  rays  of  light  from  a  ship  may  be  so  changed  in 
direction,  as  they  proceed  through  the  air,  that  the  ob- 
server will  behold  both  erect  and  inverted  images  above 
the  real  object. 

453.  ERECT  AND   INVERTED  IMAGES   ABOVE   THE 
OBJECT.    The  annexed  figure  will  aid  us  in  perceiving1 
how  erect  images  are  caused. 

What  is  said  respecting  the  cause  of  mirage  1 
Into  what  classes  may  the  phenomena  be  divided  1 
Explain  the  mirage  of  Dover  Castle. 

To  what  is  attributed  the  phenomena  of  erect  and  inverted  images  above 
the  object  1 
What  is  the  state  of  the  atmosphere  as  regards  temperature  at  such  timeit 


OPTICAL    PHENOMENA. 

Fig.  28. 


ERECT  IMAGE  ABOVE   THE  OBJECT. 


Let  D  be  a  ship,  seen  in  the  horizon  in  its  true  posi- 
tion, by  the  direct  rays  n  P,  m  P,  coming  to  the  eye  at  P, 
through  the  stratum  of  air  of  uniform  density,  lying  be- 
tween the  eye  and  the  ship.  Let  1-2,  2-3,  3-4,  &c.,  be 
parallel  strata  of  the  atmosphere,  decreasing  in  density 
from  2  to  6  ;  and  n  r  and  m  s,  rays  of  light,  proceeding 
upwards  from  the  top  and  bottom  of  the  ship.  As  these 
rays  at  r  and  5  pass  from  the  first  stratum  into  the  sec- 
ond, which  is  rarer,  they  are  bent  downwards,  or  from  the 
perpendicular,  according  to  a  well-known  law  of  optics, 
(C.  706,)  and  this  change  in  direction  continually  occurs 
as  they  pass  successively  into  strata  still  more  and  more 
rare ;  until  at  last,  as  at  x  and  y,  they  meet  the  next 
superior  stratum  so  obliquely,  that  they  are  unable  to 
enter  it,  and  are  then  totally  reflected  from  the  lower 
surfaces  of  strata  4  and  5,  at  the  points  x  and  y. 

The  rays,  on  their  return,  are  now  refracted  down- 
wards, or  towards  the  perpendicular,  (C.  706,)  in  pass- 
ing from  the  rarer  into  the  denser  media,  and  converge 
to  the  eye  at  P,  which  sees  the  vessel  in  the  direction 
of  the  last  refracted  rays.  The  ship  D  is  therefore 
beheld  at  D2  by  the  rays  Pr2  w2,  and  Ps2  m\ 


Explain  from  figure  26.  the  phenomenon  of  an  erect  image  above  the  oh- 


MAGNIFIED    IMAGES. 


185 


454.  In  figure  28.,  the  upper  ray  before  reflection  is  the 
upper  ray  after  reflection,  and  the  image  consequently 
appears  erect ;  but  if,  as  in  figure  29.,  the  rays  cross  each 


Fig.  29. 


INVERTED   IMAGE   ABOVE   THE    OBJECT. 


other  before  they  reach  the  eye  at  P,  then  the  image 
will  appear  inverted,  as  is  evident  from  the  inspection  of 
the  figure. 

Under  peculiar  circumstances  it  may  happen,  that  of 
two  sets  of  rays,  one  from  the  top  and  the  other  from 
the  bottom  of  an  object,  some  may  cross  each  other 
before  they  meet  the  eye  and  some  may  not ;  and  then 
both  erect  and  inverted  images  will  be  seen  at  the  same 


455.  MAGNIFIED  IMAGES.  The  real  object  in  figures 
28.  and  29.,  is  seen  through  the  horizontal  strata,  under 
the  visual  angle  n  P  m.  If  n*  Pm2,  the  angle  under 
which  the  image  is  seen,  is  greater  than  nPm,  the  image 
will  be  magnified  (C.  746)  in  the  direction  of  its  length  ; 
and  if  an  increase  of  the  lateral  visual  angle  occurs  at 
the  same  time,  then  the  image  will  be  likewise  magni- 

Explain  from  figure  29.  the  phenomenon  of  an  inverted  image  above  the 
object. 

When  may  both  images  be  seen  at  the  same  time  ? 
When  may  the  image  be  magnified  both  vertically  ana  nonzontaU^I 


186 


OPTICAL    PHENOMENA. 


fied  in  breadth,  and  will  appear  as  if  seen  through  a 
telescope. 

The  mirage  at  Hastings  was  probably  due  to  this 
cause. 

That  such  a  lateral  displacement  is  possible,  is  evi- 
dent from  the  remarkable  mirage  beheld  by  Messrs. 
Soret  and  Jurine,  on  the  Lake  of  Geneva,  in  Sept.  1818. 
and  which  is  shown  in  figure  30. 

Fig.  30. 


LATERAL  MtRAGB. 


456.  The  curve  ABC  represents  the  east  bank  of  the 
lake.  A  boat,  with  all  her  sails  set,  was  at  P,  advanc- 
ing towards  Geneva,  and  was  seen,  by  the  aid  of  a  tel- 
escope, in  the  direction  of  G  P,  from  Jurine's  house,  at 
the  distance  of  six  miles.  As  the  boat  successively  oc- 
cupied the  positions  M  N  R,  a  lateral  image  was  clearly 
seen  at  the  corresponding  points  M'  N'  R',  approaching 
with  the  boat,  but  appearing  to  recede  to  the  left  of  G 
P,  while  the  boat  receded  to  the  right.  When  the  sun 
shone  full  upon  the  sails,  the  image  was  visible  to  the 
naked  eye. 


Descv'be  the  lateral  mirage  seen  at  Geneva  (figure  30.),  and  explain  itt 
cause. 


IMAGES    BELOW   THE    OBJECT.  187 

The  direction  of  the  sun's  rays  at  the  time  of  obser- 
vation, is  shown  by  the  arrow  F  D. 

457.  This  curious  phenomenon  is  supposed  to  have 
been  caused  in  the  following  manner.     The  air  at  M  N 
R,  had  been  in  the  shade  all  the  morning,  while  that  at 
M'  N'  R'  had  been  warmed  by  the  sun ;  the  two  por- 
tions  therefore  were  of  different  densities,  and  the  sur- 
face which  separated  the  warm  air  from  the  cold  was 
probably  vertical.     The  rays  of  light  proceeding  from 
the  boat  might,  in  this  case,  fall  upon  the  vertical  sur- 
face of  the  warm  stratum  as  upon  a  mirror ^  (Art.  461,) 
and  being  thence  reflected  to  the  eye  of  the  observer,  an 
image  of  the  boat  would  appear  behind  this  surface. 
Thus,  if  L  I  O  was  a  part  of  this  surface,  the  ray  M  I 
from  the  boat  at  M  might  be  reflected  from  it  in  the 
direction  I  G,  and  an  image  would  then  be  seen  by  an 
observer  at  G,  in  the  direction  G  I  M'. 

458.  IMAGES   BELOW  THE  OBJECT.     Illusions,  like 
those  which  were  seen  by  the  French  army,  arise  from 
a  condition  of  the  atmosphere  exactly  the  reverse  of 
that  which  occasions  the  images  to  appear  above  the 
object. 

Upon  the  arid  plains  of  Asia  and  Africa,  when  the 
sand  is  intensely  heated,  the  temperature  of  the  air  de- 
creases and  its  density  increases  from  the  surface  up 
wards  to  a  certain  height,  where  it  is  nearly  uniform. 
A  ray  of  light,  therefore,  proceeding  from  an  elevated 
object  towards  the  ground,  must  necessarily  pass  through 
strata  of  decreasing  density,  and  will  consequently,  upon 
principles  already  explained,  after  a  number  of  refrac- 
tions be  reflected  upwards,  causing  images  to  be  seen 
below  their  objects. 

459.  For  the  sake  of  illustration,  suppose  (fig.  31.)  that 
1-2,  2-3.  3-4,  4-5,  &c.,  are  atmospheric  strata  decreas- 
ing in  density  from  the  height,  6,  to  the  surface  of  the 
ground  at  1.     Let  D  represent  a  palm  tree,  which  is 
seen  in  its  true  position  by  the  eye  at  P,  through  an 
atmosphere  of  uniform  density.     The  rays  of  light  e  a, 

Under  what  circumstances  are  images  seen  below  the  object1? 

What  is  then  the  state  of  the  atmosphere,  in  respect  to  temperature  1 


INVERTED   IMAGE   BELOW   THE    OBJECT. 


c  6,  which  proceed  from  the  top  and  bottom  of  the  tree, 
in  passing  successively  from  denser  into  rarer  media, 
will  be  constantly  bent  upwards,  until  at  last  they  suffer 
total  reflection  at  y  and  x ;  and  then,  crossing  each 
othei  t  are  again  refracted  through  the  upper  strata  con- 
verging to  the  eye  at  P.  An  inverted  image  D2  will 
therefore  be  seen  below  the  real  object  in  the  direction 
of  the  tost  refracted  rays  P  62  c2  and  P  a2  e2. 

460.  The  observer  is  led  to  imagine  these  images  in 
the  midst  of  a  lake  from  the  circumstance,  that  the  as- 
cending currents  of  warm  air,  mixing  with  the  colder 
strata,  impart  a  tremulous  motion  to  the  images  seen 
through  them  ;  and  thus  they  appear  to  be  agitated,  as 
if  floating  upon  a  slightly  ruffled  surface.     A  difference 
of  three  or  four  degrees  in  temperature  is  sufficient  to 
occasion  appearances  of  this  kind. 

461.  IMAGES  PRODUCED  BY  REFLECTION.    It  is  prob- 
able that  the  mirage  is  sometimes  produced  by  reflection 
only,  as  from  a.  plane  mirror,  and  the  instance  witnessed 

Illustrate  from  figure  31. 


REFLECTED    IMAGES.  189 

by  Oapt.  Mundy  when  travelling  in  India,  and  thus 
related  in  his  Journal,  may  have  proceeded  from  this 
cause.  "  A  deep,  precipitous  valley  below  us,  at  the  bot- 
tom of  which  I  had  seen  one  or  two  miserable  villages 
in  the  morning,  bore  in  the  evening  a  complete  resem- 
blance to  a  beautiful  lake  ;  the  vapor  which  played  the 
part  of  water  ascending  nearly  half  way  up  the  sides  of 
the  vale,  and  on  its  bright  surface  trees  and  rocks  being 
distinctly  reflected.  I  had  not  been  long  contemplating 
this  phenomenon,  before  a  sudden  storm  came  on,  and 
dropped  a  curtain  of  clouds  over  the  scene." 

The  fata  morgana  is  attributed  to  the  reflection  of 
the  rays  of  light  from  the  surface  of  the  sea  and  vapor. 

462.  The  reflecting  surface  of  a  stratum  of  air  may 
possibly  at  times  possess  a  concave  form,  so  as  to  present 
a  magnified  image  of  the  object.  (C.  732.)    In  this  way, 
the  gigantic  images  seen  by  Dr.  Tschudi  were  probably 
produced  ;  the  reflecting  surface  of  the  stratum  of  air 
being  nearly  vertical. 

463.  Another  instance  of  this  kind  occurred,  during 
the  last  war  with  England,  when  Commodore  Hardy 
was  lying  off  Boston.     A  figure  of  a  sailor  of  a  colossal 
size,  was  seen  by  his  whole  ship's  crew  reflected  in  the 
heavens,  during  a  peculiar  state  of  the  atmosphere. 

464.  To  the  same  cause  must  be  attributed  the  ex- 
traordinary phenomenon,  which  occurred  in  the  parish 
of  Migne  in  France,  on  the  17th  of  December,  1826.    It 
was  Sunday,  and  3000  persons  were  engaged  in  the 
exercises  of  the  Jubilee.     As  a  part  of  the  ceremony,  a 
large  red  cross,  twenty-five  feet  high,  was  planted  be- 
side the  church,  in  the  open  air.     Towards  the  close  of 
the  day,  while  one  of  the  clergy  was  addressing  the 
multitude,  and  reminding  them  of  the  miraculous  cross 
beheld  in  the  sky  by  Constantine  and  his  army,  a  cross 
was  seen  at  that  moment  in  the  heavens,  directly  before 
the  porch  of  the  church,  and  at  the  height  of  two  hun- 

Explain  in  what  mamssr  images  are  produced  by  reflection. 
Are  they  ever  magnified  1 
How  is  this  accounted  forl 
Give  instances. 


190 


OPTICAL    PHENOMENA. 


dred  feet  above  the  ground.  Its  length  was  nearly  one 
hundred  and  forty  feet,  its  breadth  from  three  to  four, 
and  it  shone  with  a  bright  silvery  hue,  tinged  with  red. 

The  assembled  multitude  were  struck  with  awe,  many 
regarding  it  as  a  miracle,  and  such  was  the  extraordi- 
nary sensation  produced  throughout  the  country,  that 
a  committee  was  appointed  to  investigate  this  phenome- 
non. 

From  the  circumstances  detailed  in  their  report  it  is 
evident,  that  the  cross  in  the  sky  was  the  magnified 
image  of  the  cross  before  the  church,  and  reflected  from 
the  concave  surface  of  some  atmospheric  mirror.  The 
image  possessed  exactly  the  shape  and  proportions  of  the 
wooden  cross,  it  was  tinged  with  the  same  color,  and 
the  state  of  the  air  at  the  time  was  favorable  to  the  for- 
mation of  such  images. 

465.  SPECTRE  OF  THE  BROCKEN.  The  gigantic 
spectre  which  is  supposed  to  haunt  the  Hartz  mountains 
in  Hanover,  and  is  seen  at  sunrise  from  the  Brocken,  the 
loftiest  peak  of  the  range,  is  produced  in  a  different 
manner.  It  is  in  fact  nothing  more  than  the  shadow  of 
the  observer,  cast  upon  the  thin  vapors  then  floating  in 
the  sky.  The  out  below  represents  the  spectre,-  as  seen 
by  Mr.  Haue  and  another  person,  on  the  23d  of  May, 
1797. 


SPECTRE   OP   THE    BROCKEN. 


Standing  on  the  summit  of  the  Brocken  at  sun-*<*«. 
flow  is  the  spectre  of  the  Brocken  explained  ? 


ARTIFICIAL    MIRAGE. 


191 


they  at  first  beheld  upon  the  transparent  vapors  oppo- 
site to  the  sun,  two  human  figures  of  immense  size 
which  imitated  all  their  gestures.  In  a  short  time  they 
vanished,  but  soon  re-appeared,  and  were  joined  by  a 
third,  which  likewise  mimicked  every  motion  and  atti- 
tude of  the  observers.  Similar  phantoms  are  beheld  over 
the  lake  of  Killarney,  in  Ireland. 

A  spectacle  of  this  kind  was  observed  by  Baron  Gros, 
Secretary  of  the  French  Legation  in  Mexico,  during  his 
ascent  of  Popocatepetl,  in  April,  1834.  When  he  had 
attained  a  very  great  height,  he  distinctly  beheld,  on  the 
morning  of  the  29th  at  sunrise,  the  shadow  of  the  entire 
volcano  cast  upon  the  atmosphere.  It  appeared  as  an 
immense  circle  of  shade,  through  which  the  whole  coun- 
try below  could  be  plainly  seen,  and  was  bounded  by  a 
vapor  moving  from  north  to  south.  As  the  sun  rose  the 
shadow  descended,  becoming  more  and  more  transparent, 
and  in  the  space  of  two  or  three  minutes  was  entirely 
dispersed. 

466.  ARTIFICIAL  MIRAGE.  The  phenomena  of  the 
mirage  have,  been  artificially  produced  by  Dr.  Brewster, 
in  the  following  manner. 

Fig.  33. 


ARTIFICIAL  MIRAGB. 


A  B  C  D,  figure  33.,  represents  a  trough  with  glass 
sides,  A  D,  C  B,  and  filled  with  water  up  to  the  level 
A  B.     If  a  hot  iron  is  held  near  the  surface  of  the  water 
the  heat  descends,  and  a  change  takes  place  in  the  den- 
Describe  this  phenomenon. 
What  phenomenon  was  beheld  by  Baron  Gros  1 
In  what  manner  may  an  artificial  mirage  be  produced  7 


192  OPTICAL    PHENOMENA. 

sity  of  the  fluid,  the  density  increasing  from  the  surface 
to  the  bottom.  When  the  heat  has  almost  reached  the 
bottom,  if  a  small  object,  as  a  toy-ship,  is  then  placed  at 
S,  the  eye  at  E  will  see  an  inverted  image  of  the  ship 
at  S'  and  an  erect  one  at  S":  an  appearance  similar  to 
the  mirage  observed  by  Dr.  Vince. 


CHAPTER  IV. 

OF   CORONAS   AND   HALOES. 

467.  CORONAS.     When  light,  gauze-like  clouds  float 
before  the  sun  and  moon,  their  disks  are  sometimes 
seen   encircled  by  one   or  more   colored   rings,  which 
are  termed  coronas  or  crowns.    This  appearance  is  more 
frequently  beheld  about  the  moon ;  for  the  eye  is  usually 
too  much  dazzled  by  the  brilliancy  of  the  sun  to  discern 
the  hues  that  surround  its  orb.     To  observe  them  with 
distinctness,  they  must  be  viewed  by  reflection  from  a 
blackened  mirror,  which  tempers  the  vividness  of  the 
solar  rays. 

468.  If  the  coronas  are  complete,  the  rings  are  each 
composed  of  several  concentric  circles':  the  first,  count- 
ing from  the  disk,  is  of  a  deep  blue,  the  second  white, 
and  the  third  red. 

These  three  circles  constitute  the  first  ring.  In  the 
second  the  order  of  colored  circles,  reckoning  the  same 
way,  is  purple,  blue,  green,  pale  yellow  and  red. 

469.  Rarely,  however,  are  coronas  thus  perfect ;  for 
more  often   blue  mingled  with  white  is  observed  near 
the  disk ;  this  is  followed  by  a  red  ring,  its  inner  mar- 
gin clearly  denned,  but  its  outer  limit  blended  with- the 

Of  what  does  chapter  fourth  treat  1 

What  are  coronas "? 

Around  what  orb  are  they  most  frequently  seen  1 

What  is  the  appearance  of  coronas  when  complete  1 

Are  they  usually  perfect  1 


OF    CORONAS    AND    HALOES.  193 

succeeding  circles.  If  beyond  this,  a  second  red  ring 
is  seen,  a  green  circle  occupies  the  interval  between  the 
two.  Kaemtz  discovered,  that  the  distance  of  this  latter 
circle  from  the  centre  of  the  sun  varied,  according  to 
the  state  of  the  clouds  and  atmosphere,  from  one  to 
four  degrees. 

470.  ORIGIN.     According  to  Fraunhofer,  coronas  are 
caused  by  the  diffraction  of  light ;  by  which  is  under- 
stood the  change  that  a  ray  of  light  undergoes  in  pass- 
ing across  the  edge  of  an  interposed  body,  in  conse- 
quence of  which  it  is  decomposed  into  the  seven  primary 
colors,  as  if  transmitted  through  a  prism. 

471.  This  effect  may  be  seen  if  a  ray  of  light  is  ad- 
mitted into  a  darkened   room,  through   a   very  small 
opening,  as  a  pin-hole,  and  a  knife-blade  placed  across 
the  ray.     If  the  shadow  of  the  blade  is  now  received 
upon   a  white  screen,  fringes  of  colored  light  will  be 
observed  on  each  side  of  the  shadow,  arranged  in  the 
order  of  the  prismatic  hues,  commencing  with  the  blue 
and  terminating  with  the  red.     The  manner  in  which 
coronas  are  produced  by  diffraction,   requires  a  some- 
what extended  explanation.    If  we  cut  a  fine  slit  with  a 
knife  in  a  card,  and  view  through  the  opening  any  lumin- 
ous object,  as  a  candle,  we  shall  perceive  on  each  side 
of  the  aperture  a  row  of  colored  images,  which  are  those 
of  the  candle.     Each  of  these  images  possesses  all  the 
colors  of  the  spectrum,  and  the  order  of  their  tints  is 
the  same,  the  violet  being  nearest  to  the  aperture,  and 
the  red  the  most  distant  from  it.  t 

472.  The  images  will  be  more  numerous  and  brilliant. 
if  instead  of  &  single  opening,  a  system  of  many  apertures 
is  arranged,  equal  in  size,  and  equally  distant  from  each 
other.     This  may  be  effected  by  ruling  upon  a  piece  of 
glass  with  the  point  of  a  diamond,  fine,  parallel,  and 
equi-distant  lines,  several  hundreds  in  an  inch.    If  in  a 
darkened  room  we  look  through  a  plate  of  glass  thus 


What  is  the  opinion  of  Fraunhofer  respecting  the  cause  of  coronas  1 

Explain  diffraction. 

How  may  this  phenomenon  be  exhibited  7 

9 


194  OPTICAL    PHENOMENA. 

prepared,  at  a  small  opening  in  the  window-shutter 
through  which  the  sun-light  comes,  a  row  of  prismatic 
images  of  this  opening  will  be  seen  on  each  side  of  it ; 
their  direction  being  perpendicular  to  the  lines. 

Thus,  if  figure  34.   repre-  Fig-  **• 

sents  such  a  plate  of  glass, 
the  white  lines  being  the  ruled 
lines,  and  the  aperture  is  view- 
ed through  the  point,  L,  a 
row  of  images  will  be  seen  on 
each  side  of  L  at  the  corres- 
ponding points,  a  a,  b  b,  c  c, 
and  in  a  line  perpendicular  to 
the  ruled  lines.  The  tints  PRISMATIC  IMAGES. 

are  in  the  same  order  as  in  the 

first  experiment,  and  are  caused  by  the  decomposition 
which  the  rays  of  light  undergo  in  passing  by  the  edges 
of  the  unruled  intervals  of  glass. 

If  another  series  of  parallel  lines  is  drawn  at  right 
angles  to  the  first;  a  second  row  of  colored  images  will 
start  up  in  a  direction  perpendicular  to  that  of  the  first 
images  ;  and,  if  the  ruled  lines  in  both  series  are  equally 
distant  from  each  other,  the  first  set  of  images  a  a  a  a, 
will  all  be  situated  at  the  same  distance  from  L,  each 
one  on  the  middle  of  the  side  of  a  square,  whose  centre 
is  L.  The  same  will  be  true  of  the  second  set  b  b  b  b, 
and  so  of  the  rest. 

If  there  were  three  series  of  ruled  lines,  equally  in- 
clined to  each  other,  they  would  form  regular  six-sided 
figures  or  hexagons,  and  the  images  would  be  found  on 
the  sides  of  hexagons  as  in  figure  35.,  where  the  white 
dots  represent  the  places  of  the  images. 

473.  Now  it  is  evident,  the  number  of  series  may  be 
so  increased,  that  the  figures  formed  by  the  lines  shall 
have  so  many  sides  as  not  to  differ  essentially  from 
circles  ;  and  then  the  colored  images  would  touch  each 


Show,  by  the  aid  of  figures  34.,  35.  and  36.,  the  peculiar  manner  in 
which  diffraction  operates  so  aa  to  produce  coronas. 


OF   CORONAS    AND   HALOES.  195 

other,  forming  rings  of  pris-  F'g-  &• 

matic  images  around  the  "" 
luminous  point,  the  blue 
being  the  innermost  color, 
and  the  red  the  outermost. 
Thus,  for  instance,  if  in  fig- 
ure 35.,  the  number  of  series 
was  so  multiplied,  the  larg- 
est hexagon  would  be  chan- 
ged into  the  limiting  circle, 
and  the  images,  a  a,  b  b,  c  c, 
&c.,  would  form  prismatic 
circles  around  L  as  a  common 
centre. 

474.  These   rings   appear  in  great  beauty  when  a 
luminous  object  is  viewed  through  a  plate  of  ruled  glass, 
the  lines  forming  concentric  circles,  many  hundreds,  and 
even  thousands  being  contained  in  an  inch  ;  the  distance, 
number,  and  brilliancy  of  the  rings  increasing  with  the 
fineness  of  the  lines,  and  the  narrowness  of  the  trans- 
parent intervals. 

475.  Similar   prismatic  rings  are   beheld,  whenever 
the  transparent  intervals  through  which  the  object  is 
seen  are  grouped  symmetrically  around  a  point.     Thua 
Fraunhofer,  in  looking  at  a  luminous  object  through  a 
number  of  small  glass  balls  of  equal  size,  placed  be- 
tween two  parallel  plates  of  glass,  saw  it  surrounded  by 
several  colored  circles  or  coronas.     Nor  is  this  surpris- 
ing, for  the  apertures  between  the  balls  through  which 
the  light  came,  are  arranged  concentrically  around  a 
point,  like  the  transparent  intervals  in  the  case  of  the 
circularly  ruled  lines.     This  will  be  seen  by  a  reference 
to  figure  36.,  where  (he  dark  circles  represent  the  balls, 
and  the  light  parts  between  them  the  intervals  through 
which  the  rays  come  from  the  luminous  object,  which 
is  supposed  to  be  situated  behind  the  figure.     Now  it  is 
obvious  at  a  glance,  if  the  eye  is  fixed  upon  B,  that  the 
intervals  lie  in  the  circumferences  of  circles,  whose  com- 


196  OPTICAL    PHENOMENA. 

mon  tzntre   is   the  middle  Flg- 3S- 

point  of  B.     If  we  look  at  A, 

the   intervals   are  arranged 

around  this  ball  in  a  similar 

way,  and  so  of  any  other 

ball.     In  viewing,  therefore, 

a  bright  object  through  the 

balls,  it  should  exhibit  the 

same  appearances  as  if  seen 

through  the  circularly  ruled 

glass,  and    this    is    found    tO  GLASS  BALLS. 

be  the  case. 

476.  Now  the  globules  of  vapor,  of  which  fogs  and 
clouds  are  composed,  are  arranged  in  a  similar  manner 
throughout  the  atmosphere,  and  act  upon  the  light  of 
the  sun  and  moon  as  if  they  were  so  many  small  glass 
balls.     When,  therefore,  the  rays  of  the  moon,  for  in- 
stance, reach  the  eye  of  the  observer,  after  passing  be- 
tween the  particles  of  light,  interposing  vapors,  he  will 
often  see  her  orb  surrounded  by  beautiful  coronas,  glow- 
ing with  the  rich  colors  of  the  spectrum. 

477.  Coronas   are  only  seen  when  the  globules  of 
vapor  ar-e  comparatively  few,  and  are  of  equal  size.     If 
they  are  too  numerous  a  dense  cloud  is  formed,  and  the 
intervals  being  closed  by  the  globules,  no  rays  can  pass 
through  them.     If  they  avefew  in  member  but  differ  in 
size,  then  the  intervals  are  not  symmetrically  arranged, 
and  the  sun  or  moon  will  appear  surrounded  by  a  glory, 
or  bright  circle  of  white  light. 

The  distance  of  coronas  from  the  luminous  body  is 
not  always  the  same.  The  smaller  the  particles,  the 
greater  is  the  diameter  of  the  rings. 

478.  When  white   clouds,  having    the   form  of  the 
cirro-cumulus,    float    near   the    sun,    bright,   prismatic 
colors  are  often  seen,  by  the  aid  of  a  blackened  mirror, 

When  are  coronas  only  seen  ? 

What  is  the  result  when  the  particle's  of  vapor  differ  in  size? 
Why  will  the  rings  vary  in  magnitude? 

What  is  said  respecting  the  edges  of  cirro-cumulus  and  cumulus  cloud,*, 
when  passing  near  the  sun  and  moon  ? 


ANTHELIA.  197 

fringing  the  edges  that  are  parallel  to  the  horizon.  The 
fringes  are  generally  green  within,  bordered  by  two  red 
lines. 

If  the  air  is  pure,  and  the  moon  shines  brightly,  the 
light  and  broken  edges  of  cumulus  clouds,  as  they  pas? 
near  her  disk,  are  sometimes  seen  in  like  manner  fringed 
with  prismatic  hues,  the  purple  tint  being  the  nearest 
color,  and  the  red  the  most  distant. 

479.  ANTHELIA.    Anthelia  are  coronas  seen  by  reflec- 
tion, when  the  back  of  the  observer  is  towards  the  sun  ; 
and  are  so  called  from  the  Greek  words  anti,  opposite, 
and  helios,  the  sun. 

If  the  plain  surface  of  the  circularly  ruled  glass 
(Art.  474)  is  blackened,  and  the  luminous  object  seen 
by  reflection  upon  the  ruled  side,  its  image  will  appear 
surrounded  by  colored  rings  precisely  like  those  that 
encircle  the  object  itself,  when  viewed,  as  in  the  first 
case,  by  transmitted  light. 

In  analogy  to  this,  when  the  shadow  of  a  person  is 
cast  upon  a  stratum  of  vapor,  the  head  of  the  observer, 
under  favorable  circumstances,  is  seen  surrounded  with 
prismatic  circles. 

480.  A  beautiful  display  of  this  kind  was  witnessed 
from  the  summit  of  Mount  Lafayette,  fifteen  miles  from 
Mount  Washington,  on  the  7th  of  August,  1826.     la 
the  afternoon  of  the  day  in  question,  two  gentlemen 
were  standing  upon   this   lofty  eminence,   a    thunder- 
storm was  raging  beneath  them,  and  a  sea  of  vapor  shut 
out  the  vales  from  view.     A  light  mist  was  at  this  time 
falling,  when  suddenly  the  sun  burst  through  the  clouds 
above,  and  the  observers  saw  their  shadows  resting  upon 
the  vapor  before  them,  their  heads  surrounded  with 
brilliant,  prismatic  rings.    The  circles  were  apparently 
ten  or  twelve  feet  in  diameter,  perfectly  defined,  and  their 
tints  were  exceedingly  rich  and  vivid.  This  phenomenon 
lasted   for  the  space  of  twelve  or  fifteen  minutes,  when 
it  gradually  vanished. 

481.  In  the  polar  seas,  when  the  stratum  of  fog  that 

Define  anthelia.    Relate  the  instances  given. 


198  OPTICAL    PHENOMENA. 

rests  upon  the  ocean  rises  to  the  height  of  about  three 
hundred  feet,  a  person,  stationed  upon  the  mast  of  a 
ship,  eighty  or  a  hundred  feet  above 'the  water,  perceives 
in  the  fog  opposite  the  sun,  one  or  more  circles  around 
the  shadow  of  his  head.  They  are  all  concentric; 
their  common  centre  being  in  the  imaginary  line  drawn 
from  the  sun  through  the  eye  of  the  spectator  to  the  fog 
beyond  him.  The  number  of  circles  varies  from  one  to 
Jive,  and  when  the  sun  is  bright,  or  the  fog  thick  and 
low,  they  are  usually  numerous  and  highly  colored. 

482.  On  the  23d  of  July,  1821,  Scoresby  saw  four 
concentric  circles  around  his  head,  with  the  series  of 
colors  arranged  in  the  following  order : 

1st  circle,  white,  yellow,  red,  purple. 

2d  circle,  blue,  green,  yellow,  red,  purple. 

3d  circle,  green,  whitish,  yellowish,  red,  purple. 

4th  circle,  greenish  white. 

The  colors  of  the  first  and  second  rings  were  very 
brilliant,  those  of  the  third  faint,  and  only  seen  at  inter- 
vals, while  the  fourth  exhibited  only  a  slight  tinge  of 
green.  According  to  Scoresby,  anthelia  are  always  seen 
in  the  polar  regions  whenever  fog  and  sunshine  occur 
at  the  same  time. 

483.  Several  philosophers  have  supposed   that  an- 
thelia, or  coronas  opposite  to  the  sun,  are  caused  by 
the  passage  of  light  through  frozen  particles  of  vapor, 
but  this  phenomenon  has  frequently  occurred,  when  the 
temperature  of  the  air  was  so  high  as  to  preclude  this 
idea. 

Thus  Kaemtz  often  beheld  anthelia  upon  the  Alps, 
when  the  temperature  of  the  air  was  50°  Fah.,  at  a 
short  distance  from  the  fog.  Their  explanation  upon 
the  principle  of  diffraction  is  the  most  satisfactory,  and 
the  truth  of  this  theory  is  strongly  confirmed  by  an  ob- 
servation of  Kaemtz,  who,  on  one  occasion,  first  saw  a 
corona  when  the  cloud  was  between  himself  and  the  sun, 


What  is  the  opinion  of  some  philosophers  in  regard  to  anthelia  ? 
What  objection  can  be  urged  against  this  viewl 
|   What  fact  is  stated  by  Kaemtz  ? 


HALOES. 


199 


and  then  an  anthdion  from  the  same  cloud  when  it  was 
opposite  to  the  sun. 

HALOES. 

484.  Haloes  are  circles  of  prismatic  colors  about  the 
sun  and  moon  ;  they  differ  from  coronas  in  three  partic- 
ulars ;  first,  their  structure  is  often  more  complicated  ; 
secondly,  their  diameter  is  greater;    and  thirdly,  the 
order  of  colors  is  reversed,  the  red  being  nearest  the  lu- 
minary. 

485.  The  several  parts  of  this  phenomenon  may  be 
thus  classified,  1st,  Circles  surrounding  the  orb  which 
occupies  their  centre.     2d,  Circles  passing  through  the 
orb.    3d,  Arcs  of  circles  touching  those  of  the  first  class. 
4th,  Parhelia  and  paraselenes,  or  mock-suns  and  mock 
moons,  found  at  the  points  where  the  circles  cross  each 
other. 

486.  FACTS.     The  annex-  Fig.  37. 
ed   figure   represents   a   halo 

around  the  sun,  observed  by 
Scheiner,  in  1630.  In  the 
cut,  S  is  the  sun,  ABC  a 
circle  about  45°  in  diameter, 
and  DBF  another  circle,  its 
diameter  being  nearly  95° 
20',  the  sun  being  in  their 
common  centre.  Both  the 
circles  were  colored  like  the 
primary  rainbow,  but  the 
r§d  was  next  to  the  sun,  the 
other  colors  succeeding  in 
the  natural  order.  D  S  F  is 
a  third  whitish  circle  pass- 
ing through  the  centre  of  the  sun,  and  H  E  G  a  portion 
of  a  fourth  touching  D  E  F  at  E.  At  A,  C,  D,  and  F, 
were  mock-suns  ;  the  same  phenomena  were  seen  at  B 

What  are  haloes? 

How  do  they  differ  from  coronas  or  crowns  ? 
How  are  the  several  parts  of  the  halo  classified  ? 
Describe  the  three  haloes  recorded. 


200  OPTICAL    PHENOMENA. 

and  E.  The  mock-suns,  A  and  C,  were  of  a  purplish 
red  next  to  the  sun,  while  D  and  F  were  entirely  white , 
the  former  were  also  more  brilliant,  continuing  visible 
for  three  hours  together,  while  the  light  of  the  latter  was 
faint  and  fluctuating. 

The  mock-suns  B  and  E  were  almost  the  first  to  ap- 
pear and  the  last  to  fade,  excepting  A,  and  throughout 
the  whole  phenomenon,  which  lasted  five  hours,  they 
were  perpetually  changing  in  magnitude  and  color.  B 
was  formed  in  a  peculiar  manner,  the  halo  ABC  was 
composed  of  several  intersecting  circles,  and  at  one  of 
these  intersections  the  mock-sun  B  appeared. 

487.  On  the  9th  of  Sept.  1844,  a  halo  of  a  somewhat 
complicated  structure  was  seen  by  many  observers,  both 
at  New  Haven  and  at  Hartford,  Ct.  It  continued  for  the 
space  of  four  hours,  commencing  about  10  A.  M.  and 
ending  at  2  P.  M. 

Fig.  38. 


SOLAR   HALO. 


Its  appearance  is  shown  in  figure  38.,  where  S  repie- 
sents  the  sun,  A  B  the  ordinary  halo  of  about  45°  in  di- 
ameter,  and  C  D  a  circle  whose  centre  was  in  the  zenith 
while  its  circumference  passed  through  the  sun.  Di- 
rectly north  of  the  zenith,  upon  the  circumference  of  C 
D,  a  parhelion  appeared  at  the  intersection  of  C  D  with 
the  circles  E  F  and  G  H,  which  were  both  equal  in  size 
to  itself. 


LUNAR   HALO. 


201 


Fig.  39. 


The  halo  A  B  exhibited  at  times  bright  prismatic 
tints,  and  was  attended  by  an  ellipse  or  oval,  as  seen  in 
the  figure.  The  other  circles  were  white,  and  fainter 
according  as  they  were  situated  farther  from  the  sun. 

488.  On  the  30th  of  March  1660,  at  Dantzic,  Heve- 
lius  beheld,  about  one  o'clock  in  the  morning,  the  halo 
shown  in  figure  39.  When  first  perceived  the  moon  at 
M  was  surrounded  by 
a  complete  whitish 
circle,  ABC,  45°  in 
diameter,  while  at  A 
and  C  were  two  mock- 
moons  displaying  va- 
rious colors,  and  shoot- 
ing out  at  intervals 

Dll^SflA         (VJ          OR8*^!!* 


very  long  and  whitish 
streams  of  light.  At 
two  o'clock  the  larger 
circle,  DBF,  was  seen, 
reaching  down  to  the 
horizon,  having  a  di- 
ameter of  90°. 

The  tops  of  both  circles  were  touched  by  colored 
arches,  like  inverted  rainbows,  the  red  tint  being  next 
to  the  moon.  The  arch  at  B  was  a  part  of  a  circle  equal 
in  size  to  D  E  F,  while  that  at  E  was  a  portion  of  a 
circle  of  the  same  magnitude  as  A  B  C. 

489.  Such  is  the  general  structure  of  haloes,  and  the 
identity  that  exists  in  the  magnitude  and  arrangement 
of  the  several  parts  clearly  shows,  that  they  must  origi- 
nate in  certain  fixed  laws  ;  but  what  those  laws  are  has 
not  yet  been  fully  determined. 

490.  ORDINARY  HALO  OF  45°.    The  most  satisfacto- 
ry explanation  of  this  halo  is  that  given  by  Mariotte  and 
Dr.  Young,  who  suppose  it  to  be  caused  by  the  refrac 
tion  of  the  sun's  rays,  as  they  pass  through  crystals  cf 
frozen  vapor,  floating  in  the  upper  regions  of  the  at- 
mosphere. 


LUNAR  HALO. 


What  does  the  general  structure  of  haloes  indicate  1 
Explain  the  origin  of  the  halo  of  forty-five  degrees. 

9* 


202 


OPTICAL    PHENOMENA. 


491.  For   the  sake  of  illustration 
we  will  suppose  that  A,  figure  40.,  is 
a  crystal  of  ice  or  snow,  having  its 
refracting  angle  equal  to  60°,  which 
is  the  usual  angle  of  such  crystals, 
and  that  S  E  and  S  P  are  parallel 
sunbeams,  and  E  the  eye  of  the  ob- 
server.    The  ray,  S  P,  passes  through 
the  crystal  as  through  a  prism,  and  is 
decomposed   into  its   original  colors, 
the  greatest    amount    of  prismatic 
light  reaching  the  eye  when  the  angle 
of  deviation,  S  E  R,  is  about  twenty- 
two  degrees  and  a  half. 

492.  Now,  it  is  wrell  known,  that 
in  cold  weather  the  air  near  the  earth 
is  often  filled  with  fine  needle-shaped 

crystals  of  ice,  and  that  in  the  higher  regions  of  the 
atmosphere,  above  the  limit  of  perpetual  congelation, 
crystalized  vapor  exists  in  summer  as  well  as  in  win- 
ter. (Art.  237.) 

493.  If  we   then  suppose  a  stratum  of  ice-crystals 
floating  in  the  air  so  thin  that  the  sun  is  distinctly  seen 
through  it,  though  veiled  as  by  a  slight  mist;   an  ob- 
server will  behold   this  luminary  encircled  by  rings  of 
colored  light,  proceeding  from  those  crystals  whose  an- 
gular distance  from  the  sun  is  about  twenty-two  degrees 
and  a  half. 

The  diameter  of  this  circle  or  halo,  will  of  course  be 
nearly  45°,  and  the  red  tint  will  be  next  to  the  sun  since 
it  suffers  less  refraction  than  the  blue. 

494.  It  might  be  objected,  that  the  crystals  of  snow, 
when  floating  in  the  air,  would  not  naturally  assume 
such  positions  as  to  refract  the  light  properly  to  the 
eye  ;  but  it  can  be  proved  by  rigorous  calculations,  that 
if  the  vast  number  of  crystals  which  compose  the  stra- 
tum', take  every  possible  position,  one-half  of  the  sun- 


How  is  the  objection  answered,  that  the  crystals  of  ice  would  not  na- 
turally assume  such  a  position  as  to  refract  the  light  to  the  eye? 


HALO    OF    NINETY    DEGREES.  203 

light  will  pass  through  them  ;  and  that  one-third  of  the 
transmitted  rays  will  reach  the  eye  within  a  range  of 
one  and  a  half  degrees,  viz.,  when  the  angle  of  deviation 
S  E  R  varies  from  21°'  50'  to  23°  22'. 

Such  is  the  theory  in  regard  to  the  origin  of  the  ordi- 
nary halo,  and  the  probability  of  its  truth  is  strength- 
ened by  the  fact,  that  fine  crystals  of  ice  are  known  to 
produce  curves  and  circles  of  prismatic  light. 

495.  On  the  23d  of  March,  1845,  Prof.  Snell,  of  Am- 
herst  College,  beheld  a  most  beautiful  phenomenon.    As 
he  stood  facing  the  sun,  which  had  just  arisen,  he  ob- 
served upon  the  dead  grass  before  him  a  curved,  hori- 
zontal band  of  light,  three  or  four  feet  broad,  glowing 
with  all  the  colors  of  the  rainbow.     The  top  of  the 
curve  was  twelve  or  fifteen  feet  distant,  while  the  two 
branches  extended   several  rods  to  the  right  and  left. 
The  long  spires  of  dead  grass  were  fringed  with  frost- 
crystals,  and  the  cause  of  this  brilliant  arch  was  justly 
attributed  to  the  refraction  of  the  sun's  rays  as  they 
traversed  these  minute  prisms. 

496.  If  a  distant  light,  as  a  street-lamp,  is  viewed 
through  a  pane  of  glass  upon  which  the  vapor  of  a  room 
has  crystalized,  two  or  more  fine  haloes  "torn  be  distinctly 
seen  surrounding  the  luminous  object.     The  same  ap- 
pearances are  presented  to  the  eye  if  we  substitute  a 
plate  of  glass  upon  which  a  few  drops  of  a  saturated 
solution  of  alum  have  rapidly  crystalized. 

497.  EXTRAORDINARY  HALO  OP  90°.     The  halo  of 
ninety  degrees  is  also  "supposed  to  be  owing  to  the  re- 
fraction of  light  through  crystals  of  ice  or  snow ;  the 
crystals  being  six-sided  prisms.  (Art.  283.) 

498.  CIRCLES  PASSING  THROUGH  THE  SUN.    These 
are  often  highly  colored,  and  when  the  sun  is  near  the 
horizon,  a  portion  of  a  vertical  circle  sometimes  pre- 
sents the  appearance  of  an  upright,  luminous  column. 

What  facts  are  stated  to  show  that  minute  ice-crystals  can  prjduc* 
haloes! 

How  is  the  halo  of  ninety  degrees  caused  ? 
What  is  said  respecting  the  circles  passing  through  the  sun  1 


204  OPTICAL    PHENOMENA. 

Many  years  ago,  on  a  very  cold  morning,  there  were 
seen  at  West  Point,  above  the  sun,  vertical  columns  of 
light  of  exceeding  splendor,  tinted  with  all  the  pris- 
inatic  colors,and  surpassing  in  brilliancy  the  hues  of  the 
rainbow.  A  similar  phenomenon  was  observed  at  the 
same  place,  by  Prof.  Twining,  on  the  5th  of  Jan.  1835  ; 
but  the  prismatic  tints  were  wanting. 

On  the  2d  of  January,  1586,  an  extraordinary  display 
of  this  kind  was  seen  by  Roth,  at  Cassel.  Before  the 
sun  rose,  an  upright  column  of  light  illumined  the  sky, 
at  the  point  where  the  sun  was  about  to  appear.  Its 
breadth  was  equal  to  that  of  the  sun,  and  it  glowed  like 
a  vivid  flame.  An  image  of  the  sun  next  appeared  so 
brilliant  as  to  be  taken  for  the  orb  itself.  This  was 
immediately  followed  by  the  true  sun,  which  was  di- 
rectly succeeded  by  a  second  image.  The  luminous 
column  with  its  three  suns  was  visible  for  the  space  of 
an  hour ;  the  three  suns  were  exactly  similar,  only  the 
true  one  was  the  brightest. 

499.  A  similar  phenomenon  was  observed  on  the  21st 
of  February,  1847,  by  Lieut.  Abert  and  his  party,  du- 
ring their  exploration  of  New  Mexico,  and  is  thus  de- 
scribed by  Abert  in  the  official  report  of  the  expedition : 

"  The  snow  had  heaped  up  around  the  rest  of  the  tents 
so  that  the  inmates  were  obliged  to  desert  them,  and 
take  refuge  in  the  wagons.  About  mine,  the  wind  had 
swept  in  such  a  way  as  to  keep  open  a  path  around  it, 
although  the  snow  was  on  a  level  witji  the  ridge-pole  of 
the  tent.  We  now  broke  up  some  boards  that  were  in  the 
wagons,  and  kindled  a  little  fire.  Soon  the  sun  rose  ; 
but,  instead  of  one  sun,  we  had  three  ;  all  seemed  of 
equal  brilliancy ;  but,  as  they  continued  to  rise,  the  mid- 
dle one  only  retained  its  circular  form,  while  the  others 
shot  into  huge  columns  ofjire,  which  blended  with  the 
air  near  their  summits.  The  .breadth  of  the  columns 
was  that  of  the  sun's  apparent  diameter,  and  their  height 
about  twelve  times  the  same  diameter;  they  were  be- 
tween twenty  and  thirty  degrees  distant  from  the  sun. 
Before  the  sun  had  risen  more  than  ten  degrees  the 
phenomenon  entirely  disappeared." 


PARHELIA    AND    PARASELENE.  205 

500.  The  origin  of  these  circles,  as  well  as  of  those 
which  belong  to  the  third  class,  is  attributed  to  the 
action  of  snow-crystals  upon  the  rays  of  light,  and  phi- 
losophers have  discovered  much  ingenuity  in  framing 
hypotheses  to  account  for  these  phenomena. 

501.  PARHELIA  AND  PARASELENES.    The  images  of 
the  sun  observed  in  haloes,  are  called  parhelia,  from  the 
Greek   words  para,  near,  and  helios,  the  sun  ;    while 
those  of  the  moon  are  termed  paraselence,  from  para, 
near,  and  sclena,  the  moon  :  they  have  also  received  the 
names  of  mock-suns,  and  mock-moons.     These  images 
are  found  at  the  intersection  of  the  different  circles,  and 
are  formed  by  the  accumulation  of  light  at  these  points. 
That  such  an  increase  of  light  occurs  is  obvious :  for 
if  two  equally  bright  circles  cut  each  other,  the  place 
where  they  cross  will  be  twice  as  bright  as  the  circles 
themselves.     The  parhelia  and  paraselenae  are   tinged 
with  the  colors  of  the  ordinary  halo,  and  have  frequently 
appended  to  them  a  waving  stream  of  light. 

What  is  said  as  to  their  origin  1 
What  are  parhelia  and  paraselenae  ? 
In  what  manner  are  they  produced  1 


PART    VI. 

LUMINOUS    PHENOMENA. 

U  '  " 

CHAPTER  I. 

OF    METEORITES. 

502.  METEORITES  are  those  solid  fiery  bodies  which 
from  time  to  time  visit  the  earth,  sweeping  through  the 
sky  with  immense  velocity  in  every  direction,  and  re- 
maining visible  but  a  few  moments  ;  they  are  generally 
attended  by  a  luminous  train,  and  during  their  progress 
explosions  usually  occur,  followed  by  the  fall  of  stones, 
to  which  the  name  of  aerolites  is  given,  from  the  Greek 
words  ae'r,  atmosphere,  and  lithos,  a  stone. 

503.  FACTS.     At  noon,  on  the  7th  of 'Nov.  1462,  at 
Ensisheim,  in  Germany,  a  loud  explosion  was  heard  in 
the  air,  and  a  stone  seen  to  fall  which  buried  itself  deep 
in  the  earth.     It  weighed  260  Ibs.,  and  by  the  order  of 
the  Emperor  Maximilian,  was  suspended  in  the  church 
at  Ensisheim,  where  it  remained  until  the  French  revo- 
lution.    A  portion  of  it  is  now  in  the  Parisian  museum, 
and  another  in  the  Imperial  Cabinet  at  Vienna. 

On  the  21st  of  June,  1635,  a  fiery  mass  was  seen 
passing  over  the  Veronese  territory  with  such  velocity, 
that  the  eye  could  scarcely  follow  its  motions.  Loud 

What  is  the  subject  of  part  sixth  ? 

What  of  chapter  first  1 

Define  meteorites. 

What  are  aerolites  1 

Relate  the  account  of  the  meteorite  of  Ensisheim. 

Of  lhat  of  Verona. 


METEORITES.  207 

explosions  were  heard,  and  a  large  stone  fell  near  the 
Benedictine  Convent,  about  six  miles  from  Verona. 

504.  At  half  past  six  o'clock,  on  the  morning  of  the 
14th  of  Dec.  1807,  a  meteorite  was  seen  rushing-  from 
north  to  south,  over  Weston,  in  the  State  of  Connecti- 
cut; its  apparent  diameter  being  equal  to  one-half,  or 
two-thirds,  that  of  the  full  moon.     As  it  passed  behind 
the  clouds,  it  appeared  like  the  sun  through  a  mist,  and 
shone  with  a  mild  and  subdued  light ;  but  when  it  shot 
across   the   intervals  of  clear   sky,  the   glowing   body 
flashed  and  sparkled  like  a  firebrand  carried  against  the 
wind.    Behind  it  streamed  a  pale,  luminous  train,  taper- 
ing in  form,  and  ten  or  twelve  times  as  long  as  its  diam- 
eter. 

The  meteorite  was  visible  for  the  space  of  half  a 
minute,  and  just  as  it  vanished  gave  three,  distinct 
botmds.  About  thirty  seconds  after  its  disappearance, 
three  heavy  explosions  were  heard  like  the  reports  of  a 
cannon,  succeeded  by  a  loud,  wjiizzing  noise.  Directly 
after  the  explosions,  a  person  of  the  name  of  Prince 
heard  a  sound  resembling  that  occasioned  by  the  fall  of 
a  heavy  body,  and  upon  going  from  the  house  perceived 
a  fresh  hole  in  the  turf,  at  the  distance  of  twenty-five 
feet  from  the  door.  At  the  bottom  of  the  hole,  two  feet 
below  the  surface,  an  aerolite  was  discovered  which 
weighed  nearly  35  pounds. 

Another  mass,  which  was  dashed  to  pieces  upon  a 
rock,  was  judged,  from  the  fragments  collected,  to  have 
weighed  two  hundred  pounds.  Other  aerolites  fell  in 
various  parts  of  the  town.  The  stones,  at  the  time  of 
their  descent,  were  hot  and  crumbling,  but  gradually 
hardened  upon  exposure  to  the  air. 

505.  At  Futtypore,  in  India,  on  the  5th  of  Nov.  1814, 
a   meteorite  was   seen,  shortly   after   sunset,    shooting 
swiftly  towards  the  north-west,     It  appeared  as  a  blaze 
of  light  surrounding  a  red  globe  of  the  apparent  size  of 
the  moon.     As  it  proceeded*  on  its  course,  loud  explo- 
fiions  were  heard,  resembling  the  sound  of  distant  artil- 

Of  that  of  W«ston. 
Of  that  of  Futtypore. 


203  LUMINOUS    PHENOMENA. 

lery,  and  a  stone  fell,  which,  in  its  descent,  emitted 
sparks  like  those  proceeding  from  a  blacksmith's  forge. 
When  first  discovered,  the  aerolite  was  hot,  and  ex- 
haled a  strong  sulphurous  smell. 

On  the  llth  of  Dec.  1836,  just  before  midnight,  a  me- 
teorite of  extraordinary  size  and  brilliancy  was  seen 
over  the  village  of  Macao,  in  Brazil,  traversing  a  cloud- 
less sky.  It  burst  with  a  sharp,  loud  noise,  and  a 
shower  of  stones  fell  within  a  circle  of  ten  leagues.  The 
aerolites  varied  in  weight  from  one  pound  to  eighty,  and 
descended  with  such  force  as  to  break  through  the  roofs 
of  houses,  and  bury  themselves  deep  in  the  sand. 

506.  Dr.  Chaldni  has  compiled  an  extensive  catalogue 
of  instances  when  meteorites  have    been   seen ;   from 
which  it  appears,  that  these  extraordinary  bodies  have 
been  noticed  from  the  earliest  ages,  and  in  all  parts  of 
the  world ;  and,  since  attention  has  been  drawn  to  the 
subject,  scarcely  a  year  now  passes  without  one  or  more 
well-attested  cases  of  the  fall  of  aerolites. 

507.  SIZE  OP  METEORITES.    We  must  not  confound 
the  magnitude  of  the  meteorite  with  that  of  the  aerolite , 
for  the  latter  is  nothing  more  than  a  fragment  thrown 
off  from  the  former  and  falling  to  the  earth,  while  the 
main  body  sweeps  onwa'd  in  its  course. 

The  diameter  of  the  Weston  meteorite  was  computed 
to  be  300  feet,  and  that  of  the  meteorite  observed  by 
Mr.  Cavallo,  at  Windsor,  on  the  18th  of  Aug.  1783,  was 
calculated  by  this  gentleman  to  be  no  less  than  3210 
feet,  or  more  than  three-ffths  of  a  mile.  Mrs.  Somer- 
ville  mentions  one  that  was  estimated  to  weigh  nearly 
600,000  tons. 

508.  ALTITUDE.    The  height  of  meteorites  above  the 
earth  has  been  estimated,  and  found  to  vary  from  18  to 
70  or  80  miles.     According  to  the  calculations  of  Dr. 
Bowditch,  the  meteorite  of  Weston  approached  within  18 
miles  of  our  globe,  and  one  mentioned  by  Mrs.  Somer 


Of  that  of  Macao. 

When  and  where  have  meteorites  appeared  1 

What  is  said  regarding  their  size,  altitude  and  velocity  ? 


VELOCITY.  209 

ville,  ca'.rie  within  tlie  distance  of  25  miles.  An  ;n 
stance  is  given  by  Dr.  Halley  of  a  meteorite  that  explod- 
ed at  an  elevation  of  69  miles,  with  a  report  like  that  of 
a  cannon. 

509.  VELOCITY.     The  velocity  of  these  bodies  is  gen- 
erally somewhat  more  than  300  miles  per  minute,  though 
many  cases  have  occurred  of  far  greater  speed  ;  the  me- 
teorite just  mentioned,  that  came  within  25  miles  of  our 
earth,  moved  at  the  rate  of  1200  miles  per  minute. 

510.  If  a  body  in  the  atmosphere  is  seen  at  the  same 
time  by  two  observers  upon  the  earth  at  different  sta- 
tions, and  its  angular  elevation  taken  at  both  stations  ; 
its  height  in  miles  and  feet  is  easily  ascertained  by  the 
aid  of  trigonometry,  when  the  distance  between  the  two 
stations  is  known.     If  the  body  is  in  motion,  and  its  po- 
sition noted  at  the  moment  of  rts  appearance  and  disap- 
pearance, the  distance  it  travels  in  this  interval,  or  the 
length  of  its  visible  path  can  be  obtained,  when  its  height 
has  first  been  computed.     The  speed  is  ascertained  by 
dividing  the  length  of  the  visible  path  by  the  number  of 
seconds  during  which  the  body  is  seen.     The  magni- 
tude is  easily  obtained  by  trigonometrical  calculations, 
when  the  distance  of  the  body  and  its  angular  diameter 
is  known.    In  this  manner  computations  are  made  upon 
meteorites,  shooting-stars  and  the  aurora  borealis. 

511.  From  their  sudden  appearance  and  extreme  ve- 
locity, all  observations  upon  these  phenomena  are  liable 
to  great  inaccuracy,  and  estimates  of  magnitude,  velo- 
city and   height,  derived  from  such  observations  must 
be  received  with  much  allowance,  and  are  to  be  regard- 
ed only  as  approximations,  more  or  (less  near  to  the 
truth. 

AEROLITES. 

512.  FORM.      The  greater  number  of  aerolites,  ac- 
cording to  Schreibers,  have  always  the  same  general 
form,  which  is  that  of  an  oblique  or  slanting  pyramid. 

In  what,  manner  are  calculations  made  respecting  the  size,  altitude,  speed, 
&c.,  of  bodies  high  above  the  earth  1 
What  is  said  as  to  estimates  of  this  kind  1 
What  is  the  form  and  external  appearance  of  aerolites  1 


210 


LUMINOUS    PHENOMENA. 


They  are  also  alike  in  external  appearance,  presenting 
to  view  a  black,  shining-  crust,  as  if  the  body  had  been 
coated  with  pitch.  This  crust  is  not  greater  than  the 
two-hundredth  part  of  an  inch  in  thickness,  its  composi- 
tion is  identical  with  that  of  the  mass,  it  bears  the  marks 
of  fusion,  and  strikes  fire  with  the  flint.  When  broken, 
the  surface  of  the  fracture  displays  the  color  of  an  ask- 
grey. 

"Distinct  aerolites,"  says  Berzelius,  the  celebrated 
chemist,  "  are  frequently  so  like  one  another  in  color  and 
external  appearance,  that  we  might  believe  them  to  have 
been  struck  out  of  one  piece." 

513.  COMPOSITION.    According  to  Berzelius,  aerolites 
consist   of  eighteen   elementary  substances.      A  nine- 
teenth has  since  been  discovered,  and  perhaps  two  others. 
They  are  remarkable  for  containing  malleable  metallic 
iron,  nickel,  and  chrome.     Their  specific  gravity  varies 
from  3.35  to  4.28. 

514.  These  common  characteristics  indicate  a  com- 
mon origin,  and  this  we  are  led  to  seek  beyond  the  earth, 
inasmuch  as  the  composition  of  aerolites  is  totally  dif- 
ferent from  that  of  any  stony  mass,  forming  a  part  of  the 
crust  of  the  globe.     Malleable  metallic  iron  is  rarely,  if 
ever,  found  in  terrestrial  substances,  nickel  is  extremely 
scarce,  and  has  never  been  discovered  on  the  surface  of 
the  earth  ;  and  chrome  is,  if  possible,  still  more  rare. 

515.  It  sometimes  happens,  though  seldom,  that  the 
aerolite  consists  almost  entirely  of  metallic  iron.     On 
the  26th  of  May,  1751,  a  meteorite  burst  with  a  tremen- 
dous report,  over  Hradschina,  in  the  district  of  Agram,  in 
Upper  Sclavonia..    Two  fragments  were  seen  rushing  to 
the  earth,  the  largest  of  which  struck  deep  into  the  soil. 
This  mass  weighed  71  Ibs.,  exhibited  evident  traces  of 
fire,  and,  upon  being  analyzed,  gave,  out  of  every  100 
parts,  95.5  of  iron  and  3.5  of  nickel.     A  portion  of  the 

What  is  their  composition  1 

What  do  these  common  characteristics  indicate? 

Where  must  it  be  sought— and  why  ? 

Of  what  does  the  aerolite  sometimes  consist  1 

Give  the  instance. 


AEROLITES.  211 

iron  being  finely  polished,  and  corroded  with  acids,  a 
most  beautiful  crystaline  structure  was  revealed,  branch- 
ing in  every  direction  over  the  surface.  This  peculiarity 
oelongs  to  most  of  the  specimens  of  the  iron  of  mete- 
orites. (See  Fig.  41.) 

Fig.  41. 


CRYSTALINE   STRUCTURE   OF  THB   METEORIC   IRON  OF  TEXAS. 

(.Copy  of  an  Impression  taken  from  the  Iron.) 

516.  METEORIC  IRON.     From  the  peculiar  constitu- 
tion and  structure  of  aerolites,  we  are  enabled  to  detect 
the  meteoric  origin  of  masses  of  iron  which  are  occasion- 
ally found  scattered  over  the  surface  of  the  earth,  in  all 
quarters  of  the  globe.     For  since  they  possess  the  same 
elements  as  the  iron  of  aerolites,  combined  in  the  same 
manner,  and  as  no  such  masses  have  ever  been  taken 
from  mines,  we  must  necessarily  conclude,  that  they  were 
once  exploded  from  a  meteorite,  though  no  record  exists 
of  their  fall. 

517.  Humboldt  relates,  that  in  Mexico,  near  the  envi- 


What  structure  did  this  iron  possess  1 

Why  are  certain  masses  of  iron  supposed  to  have  a  meteoric  origin "? 


212  LUMINOUS    PHENOMENA, 

rons  of  Durango,  is  an  enormous  mass  of  malleable  iron 
and  nickel,  which  possesses  exactly  the  same  composi- 
tion as  the  fragment  that  felt  at  Agram. 

A  mass  of  'metallic  iron,  weighing  1544  Ibs.,  was  dis- 
covered by  Prof.  Pallas,  in  1771,  at  Krasnojark  in  Sibe- 
ria. It  was  regarded  by  the  Tartars  as  a  sacred  ob- 
ject, and  according  to  their  traditions  had  fallen  from 
heaven. 

The  famous  mass  of  malleable  iron  which  was  found 
in  Texas  in  1808,  and  is  now  in  the  cabinet  of  Yale 
College,  weighs  1635  Ibs.  It  contains  nickel. 

518.  During  an  expedition  in  South  Africa,  Sir  James 
Alexander  discovered,  near  the  Great  Fish  river,  a  con- 
siderable tract  of  country,  over  which  fragments  of  me- 
tallic iron  were   scattered  in   profusion ;   a  specimen 
analyzed  by  Sir  John  Herschel,  was  found  to  possess 
nickel,  thus  proving  conclusively  the  meteoric  origin  of 
the  masses. 

519.  ORIGIN  OP  METEORITES.   Natural  philosophers 
have  advanced  five  hypotheses,  to  account  for  the  origin 
of  these  extraordinary  bodies. 

•     1st.   That  they  are  ejected  from  terrestrial  volcanoes. 

2d.  That  they  are  produced  in  the  atmosphere,  being 
formed  from  the  gases  exhaled  from  the  earth. 

3d.   That  they  are  thrown  from  lunar  volcanoes. 

4/A.  That  they  are  terrestrial  comets  revolving  about 
the  earth  like  the  moon. 

5th.  That  they  are  celestial  bodies  revolving  about 
the  sun  like  the  planets,  and  encountered  by  the  earth 
in  its  annual  progress. 

520.  FIRST  HYPOTHESIS.    The  first  supposition  can- 
not be  maintained,  since  it  is  impossible  for  the  volca- 
noes of  the  globe  to  hurl  to  the  height  of  twenty  miles 
masses  of  the  size  of  meteorites  ;  besides,  the  composi- 


Illustrate  from  the  several  instances  given. 

How  many  hypotheses  have  been  advanced,  to  account  for  the  origin 
of  meteorites,  and  what  are  they  1 
What  are  the  objections  to  the  first  hypothesis  1 


ORIGIN.  213 

ftion  of  the  latter  is  entirely  different  from  all  volcanic 
products. 

521.  feEcoND  HYPOTHESIS.     The  second  is  likewise 
untenable.      Nickel,  according1   to   high    chemical   au- 
thorities, has  never  been  raised  in  vapor ;  even  under 
the  intense  heat  of  volcanoes.    A  mass  of  matter  formed 
in  the  air,  must  therefore  be  destitute  of  nickel,  an  ele- 
ment which  meteorites  invariably  possess.     Moreover, 
such  a  body,  in  its  descent,  would  fall  perpendicularly 
to  the  ground  by  the  action  of  gravity,  and  rtot  sweep 
along,  as  did  the  Weston  meteorite,  in  a  direction  nearly- 
parallel  to  the  surface  of  the  earth. 

522.  THIRD  HYPOTHESIS.    In  regard  to  the  third  hy- 
pothesis, it  has  been  shown,  by  calculations  of  La  Place, 
and   other  eminent  mathematicians,  that  a  mass  pro- 
jected from  the  moon,  with  a  velocity  of  10,660  feet  per 
second,  would  pass  beyond  the  point  of  the  moon's  at- 
traction, and  either  fall  to  the  globe  in  the  space  of  two 
days   and   a  .half  by  the  force  of  gravity,  or  revolve 
about  the  earth  like  the  moon.     It  is  not  therefore  im~ 
possible,  that  such  an  event  might  occasionally  occur  , 
but  it  is  utterly  improbable  that  meteorites  originate  in 
this  manner. 

523.  Omitting  other  objections  to  this  hypothesis,  the 
size  and  number  of  meteorites  constitute  an  insuperable 
difficulty.    It  requires  a  strong  faith  to  believe,  that  such 
masses,  as  have  been  described,  could  be  hurled  from  a 
lunar  volcano,  at  the  rate  of  not  less  that  10,000  feet  per 
second  ;  a  speed  Jive  times  greater  than  the  highest  ve- 
locity of  a  cannon-ball. 

524.  The  number  of  meteorites  must  be  also  veiy 
great ;  for  they  have  been  seen  from  the  earliest  ages 
and  in  all  inhabited  quarters  of  the  globe  occasionally 
traversing    the   heavens,  and   those  which   have  been 
noticed  are  probably  only  a  part  of  the  actual  number 
that  have  visited  the  earth.     Many  must  have  passed 
unseen  over  the  broad  expanse  of  ocean,  or  crossed  vas' 

What  to  the  second  ? 

What  argument  is  advanced  in  favor  of  the  third  hypothesis? 

What  arguments  against  it  ? 


214  LUMINOUS    PHENOMENA. 

tracts  of  uninhabited -lands,  leaving  no  trace  of  the»r 
existence,  except  those  masses  of  meteoric  iron,  whicTi 
from  time  to  time  are  brought  to  light.  If  therefore  the 
lunar  theory  is  adopted,  we  can  scarcely  avoid  the  con- 
clusion, that  the  moon  has  been  ejecting  for  ages,  so 
many,  and  such  vast  masses  of  matter,  as  must  have 
sensibly  diminished  her  bulk,  and  occasioned  derange- 
ments in  her  system — results  at  variance  with  all  obser 
vations. 

525.  FOURTH  HYPOTHESIS.    The  fourth  hypothesis, 
which  is  that  of  President  Clap,  of  Yale  College,  affords  a 
more  reasonable  explanation  of  the  phenomena  of  these 
extraordinary  bodies  than  any  of  the  preceding.     Under 
this  view  the  earth  is  supposed  to  possess  a  system  of  com- 
ets like  the  sun.     The  solar  comets  revolve  about  their 
primary  in  very  extended  orbits ;    at  one  part  of  the 
course  approaching  so  near  the  sun  as  almost  to  strike 
its  surface,  and  during  the  remainder,  sweeping  far  out 
of  sight    beyond   the  path  of  the   planets,  continuing 
invisible  for  years  and  even  ages.     In  like  manner  me- 
teorites are  supposed  to  revolve  about  the  earth  ;  their 
size  and  periods  of  revolution  being  proportioned  to  the 
smaltness  of  their  primary.     Moving  also  in  very  ellip- 
tical  or  oval  orbits,  they  are  too  distant  to  be  visible 
during  the  greater  part  of  their  course,  but  at  one  point 
of  their  path  approach  very  close  to  the  earth,  and  enter 
its  atmosphere. 

On  account  of  the  immense  velocity  of  the  meteorite, 
the  air  is  imagined  to  be  condensed  before  it  to  such  a 
degree,  that  heat  is  evolved  of  sufficient  intensity  to 
inflame  the  mass  at  its  surface,  while  during  this  com- 
bustion gases  are  generated,  which  by  their  expansive 
energy,  produce  explosions.  By  the  strength  of  this  dis- 
ruptive force,  glowing  fragments  are  detached  frorr.  the 
surface  and  fall  to  the  ground,  while  the  meteorite  itself 
passes  onward  on  its  course. 

526.  It  has  been  calculated,  that  the  velocity  of  a 


Whicti  hypothesis  affords  a  more  reasonable  explanation  3 
Explain  it  fully. 


ORIGIN.  215 

body  revolving  about  tbe  earth  nrast  not  be  less  than 
300  miles  per  minute,  nor  greater  than  420.  Were  it 
less  than  300  miles,  the  mass  would  fall  to  the  earth  by 
the  action  of  gravity ;  and  if  the  rate  exceeded  420 
miles,  it  would  pass  away  from  the  globe  and  never 
return.  Within  these  limits,  allowance  being  made  for 
the  motion  of  the  earth  in  its  orbit,  and  the  resistance 
of  the  air,  the  body  would  revolve  around  the  earth  like 
the  moon,  approaching  very  near  to  its  surface  at  stated 
periods. 

527.  In  support  of  this  hypothesis  it.  is  urged,  that  the 
velocity  of  meteorites,  in  general,  is  somewhat  more  than 
300  miles  per  minute,  though  doubtless  cases  have  oc- 
curred in  which  their  speed  was  far  greater. 

The  combustion  of  the  meteorite,  through  the  agency 
of  a  condensed  atmosphere,  is  by  no  means  improbable ; 
for  though  the  medium  in  which  it  moves  is  exceedingly 
rarefied,  yet  the  velocity  of  the  body  is  amazing ;  and 
it  can  easily  be  shown  by  calculation,  .that  from  the 
condensation  thus  effected,  an  intensity  of  heat  would 
be  developed  of  which  we  have  no  conception.  (Art. 
551.)  Moreover,  as  silica,  magnesia,  and  potassa  are 
found  in  meteorites,  it  has  been  conjectured,  that  they 
may  originally  exist  there  in  the  state  of  pure  metals  ; 
and,  that  when  Ihe  meteorite  enters  our  atmosphere, 
combustion  arises  from  the  extraordinary  affinity  of 
these  substances  for  oxygen. 

In  those  instances  where  meteorites  move  at  a 
greater  rate  than  420  miles  per  minute,  they  are  sup- 
posed either  to  revolve  about  the  sun,  and  that  the  earth 
occasionally  meets  them  in  her  annual  progress ;  or  to 
wander  through  space,  until  they  come  within  the  supe- 
rior attraction  of  some  other  orb,  and  are  then  com- 
pelled to  revolve  around  it. 


What  calculation  has  been  made  in  respect  to  a  body  revolving  about 
the  earth  ? 

What  facts  and  suggestions  are  adduced  in  support  of  Pres.  Clap's  hy- 
pothesis ? 

What  is  said  of  meteorites  moving  at  a  greater  rate  than  420  miles  per 
minute  1 


216  LUMINOUS    PHENOMENA. 

528.  FIFTH  HYPOTHESIS.  The  last  hypothesis  is  that 
of  Ohaldni,  and  is  explained  in  Art.  555.  In  this  the 
ignition  and  explosion  of  the  meteorite  are  attributed 
to  precisely  the  same  causes  as  those  assigned  in  the 
fourth  hypothesis. 


CHAPTER  II. 

OF  SHOOTING-STARS  AND  METEORIC  SHOWERS. 

529.  Shooting-stars  or  meteors  differ  from  meteorites 
in  several  particulars.    They  commonly  possess  a  supe- 
rior velocity,  and   their  altitude  is  generally  greater : 
bursting  from  the  clear  sky,  they  dart  along  the  heaven 
like  a  rocket,  consuming  themselves  in  their  course,  and 
leaving  behind  a  luminous  train,  which  gradually  van- 
ishes in  a  short  time.    Unlike  the  meteorite  they  usually 
pass  away  without  any  explosion,  and  no  portion  of  the 
body  ever  reaches  the  earth.     Besides,  they  are  far  more 
numerous  and  frequent ;  appearing  almost  every  night, 
and  at  times  descending  in  such  multitudes  that  the  heav- 
ens are  illumined  for  hours  with  their  glowing  trains. 

530.  ALTITUDE.    In  order  to  investigate  the  phenom- 
ena of    shooting-stars,  Brandes  and    Benzenberg,   two 
German    philosophers,   made  a  series  of  simultaneous 
observations  in  the  fall  of  the  year  179S.     On  six  even- 
ings, between  September  and  November,  402  shooting- 
stars  were   beheld,  and   of   these    twenty-two  were    so 
identified,  that  their  altitudes,  at  the  moment  of  their 
extinction,  could  be  readily  computed.    They  were  found 
to  be  as  follows: 

7  disappeared  at  altitudes  under  45  miles. 
'  9          "  "         between  45  and  90  miles. 

6          "  "         above  90  miles. 

Of  what  does  chapter  second  treat  ? 

Jn  what  particulars  do  shooting-stars  and  meteors  differ  from  meteorites'? 
Relate  the  account  of  the  observations  of  Brandes  and  Benzenberg,  for 
Determining  the  altitudes  of  shooting-stars  1 
Give  their  results. 


SHOOTING-STARS.  217 

The  least  and  greatest  elevations  were  six  miles  and 
me  hundred  and  forty. 

531.  In  1823,  the  investigation  was  renewed  by 
Brandes,  at  Breslau  and  the  neighboring  towns,  on  3, 
more  extended  scale.  Between  April  and  October.  18CM 
shooting-stars  were  seen  at  the  different  stations.  Out 
of  this  number,  98  were  observed  simultaneously  at 
more  than  one  station,  and  afforded  the  means  of  esti- 
mating their  respective  altitudes.  The  results  were  as 
follows: 

4  disappeared  at  altitudes  under  15  miles. 
15  between  1  *>  and  30  miles. 

22          «  «  «       30     "    45     " 

33         "  "  "      45     «    70     « 

13         "  «  «      70     «    90     " 

11  "  above     90  « 

Out  of  the  last  eleven,  two  vanished  at  an  elevatiott 
of  140  miles,  a  third  at  220  miles,  a  fourth  at  280  miles, 
and  a.  fifth  at  460  miles. 

The  height  of  four  shooting-stars  noticed  by  Profes- 
sors Loomis  and  Twining,  in  December,  1834,  varied 
from  54  miles  to  94. 

532.  Similar  observations  were  made  in  Switzerland," 
on  the  10th  of  August,  1838,  by  Wartman  and  others. 
A  part  of  the  observers  stationed  themselves  at  Geneva, 
and  the  rest  at  Planchettes,  a  village  about  sixty  miles 
to  the  north-east  of  that  city.     In    the  space  of  seven 
hours  and  a  half,  381  shooting-stars  were  seen  at  Gene- 
va, and  in  five  hours  and  a  half  104  at  Planchettes.    All 
the  circumstances  attending  their  appearance  were  care- 
fully noted,  and  their  average  height  was  computed  at 
five  hundred  and  fifty  miles. 

533.  VELOCITY.     In  the  first  series  of  observations 
made  by  Brandes  and  Benzenberg,  only  two  shooting- 
stars  afforded  the  means  of  determining  their  speed; 
one  possessed  a  velocity  of  1500  miles  per  minute,  and 

Give  those  of  Loomis  and  Twining. 
Give  those  of  Wartman,  at  Geneva. 

What  is  their  velocity  according  to  the  observations  of  Brandes  and 
Benzenberg,  and  Q,uetelel  7 

to 


218  LUMINOUS    PHENOMENA. 

that  of  the  other  was  between  1020  and  1260  miles  per 
minute. 

In  the  second  series,  undertaken  in  1823,  the  estimat- 
ed rate  of  motion  varied  between  1080  and  2160  miles 
per  minute.  At  Belgium,  in  1824,  M.  Q,uetelet  obtain- 
ed observations  upon  six  of  these  singular  bodies,  from 
which  he  was  enabled  to  deduce  their  respective  veloci- 
ties, which  were  found  to  range  from  600  to  1500  miles 
per  minute. 

534.  COURSE.     Of  thirty-six  stars,  whose  paths  were 
ascertained  by  Brandes,  the  motion  in  twenty-six  cases 
was  downward,  in  one  horizontal,  and  in  the  remaining 
nine,  more  or  less  upward  ;  nor  did  (hey  always  move 
in  straight  lines  ;  for  the  paths  of  some  were  curved. 
either  upwards  or  sideways ;   while  others  proceeded  in 
a  serpentine  course.     Their  general  direction  was  from 
north-east  to  south-west.     Several  examples  have  been 
given  by  Chaldni,  where  the  luminous  body  described  a 
semicircle,  first  rising  and  then  falling. 

535.  MAGNITUDE.     The  size  of  shooting-stars  is  va- 
riable.    Fire-balls,  which  are  regarded  as  nothing  more 
than    large   meteors,    have    sometimes   appeared   of  a 
"magnitude  almost  incredible.     During  the  remarkable 
shower  of  meteors,  on  the  12th  and  13th  of  November, 
1833,  luminous  globes,  apparently  as  large  as  Jupiter 
and  Venus,  were  seen  darting  through  the  air  in  all 
directions.     About  three  o'clock  on  the  morning  of  the 
13th,  a  splendid  body  which  appeared  equal  in  size  to 
the  lull  moon,  swept  across  the  heaven  from  east  lowest. 
If  the  distance  of  this  meteor  was  only  eleven  miles,  its 
diameter  must  have  been  528  feet,  or  one  tenth  of  a 
mile.     Amid  the  shower  of  stars  that  occurred  in  1799, 
meteors  were  observed  by  Humboldt,  apparently  twice 
the  size  of  the  moon. 

536.  On  the  evening  of  the  18th  of  May,  1838,  a 
meteor  of  extraordinary  magnitude   passed   over   the 


What  is  said  respecting  the  course  of  shooting-stars  ? 

What  of  their  magnitude  1 

Relate  the  account  of  the  meteor  of  the  18th  of  May. 


SHOOTING  STARS.  219 

Northern  States  and  a  part  of  Canada.  From  the  facts 
which  he  collected,  Prof.  Loomis  estimated  its  diame- 
ter at  1320  yards,  or  three  quarters  of  a  mile.  Its  velo- 
city was  computed  by  this  gentleman  to  be  nearly  2100 
miles  a  minute,  its  height  to  be  30  miles,  and  the 
length  of  its  path  200  miles.  The  meteor  was  followed 
by  a  train  of  inconsiderable  extent,  probably  formed 
of  the  detached  portions  of  the  body  which  fell  be- 
hind. 

537.  SPLENDOR.      At  times  these  luminous  bodies 
present  a  spectacle    of   surpassing  beauty,  from  their 
brilliant  coruscations,  extended  trains,  and  rich  diversity 
of  colors.     During  the  month  of  April,  1832,  a  globular 
ball  of  fire,  apparently  afoot  in  diameter,  passed  over 
Torhut,   in   India,   early  in   the  morning.      Behind  it 
streamed  a  train  of  dazzling  light,  which  appeared  to  be 
several  yards  in  length.     The  meteor  illumined   the 
surrounding  country  to  a  great  distance,  and  after  re- 
maining visible  for  the  space  of  five  seconds,  exploded 
without  noise,  like  a  rocket,  throwing   out  numerous 
coruscations  of  intense  splendor. 

In  May  of  the  same  year,  and  at  the  same  place,  a 
similar  body  was  seen  moving  rapidly  through  the  air, 
from  north  to  south.  It  glowed  with  a  brilliant  mixture 
of  green  and  blue  light,  and  vanished  in  about  three 
seconds,  leaving  a  luminous  train  of  great  length. 

538.  During  the  nights  of  the  9th  and  10th  of  August, 
1839,  many  shooting-stars  of  singular  beauty  were  seen 
by  Mr.  E.  C.  Herrick,  of  New  Haven.     One  flashed 
\vith  a,  golden  green  light,  and  another  sparkled  with 
green  and  blue.     Meteors  entirely  green  have  at  times 
been  noticed.     A  meteor  which  swept  over  Kensington, 
near  London,  in  1839,  as  brilliant  as  Jupiter  and  ap- 
parently of  greater  size,  presented  the  rare  combination 
of  white  light  in  the  mass,  with  one  edge  red  and  the 
opposite  of  a  deep  blue  or  purple. 

On  the  morning  of  the  13th  of  November,  1833,  n 
most  brilliant  meteor  was  seen  by  Prof.  Twining,  de- 

What  is  sajd  of  their  splendor  1 


220  LUMINOUS    PHENOMENA. 

scending  towards  the  earth  with  majestic  rapidity.  Its 
apparent  size  was  one-fifth  that  of  the  moon,  and  its 
color  a  deep  red.  It  vanished  when  near  the  ground, 
leaving  behind  a  fiery  train  of  the  same  hue,  excepting 
that  it  displayed  the  prismatic  tints,  especially  at  the 
point  where  the  meteor  expired. 

539.  The  usual  color  of  meteors  is  that  of  a  phospho- 
ric white  tinged  with  red.     The  trains  generally  vanish 
in  a  few  seconds,  but  they  have  been  known  to  last  for 
the  space  of  seven  minutes,  and  even  fifteen.     Their 
light  (as  we  have  just  seen)  is  not  invariably  of  one  hue, 
for  at  times  it  presents  to  the  eye  all  the  rich  tints  of  the 
rainbow. 

METEORIC    SHOWERS. 

540.  The  wondrous  display  of  meteors  in  1833,  drew 
the  attention  of  philosophers  to  the  subject  of  shooting 
stars,  and,  from  the  results  of  subsequent  researches  and 
observations,  there  is  now  reason  to  believe,  that  certain 
epochs  exist  when  these  luminous  bodies  appear  in  greater 
numbers  than  usual,  and  that  sometimes  at  the  return 
of  these  periods  they  literally  descend  to  the  earth  in 
showers.     The  best  ascertained  periods  are  those  of  the 
12th  and  13th  of  November,  and  the  9th  and  10th  of 
August. 

541.  NOVEMBER   EPOCH.     On   the  morning  of  the 
12th  of  November,  1799,  an  extraordinary  display  of  this 
nature  was  seen  by  Humboldt  and  Bonpland,  at  Cuma- 
na,  in  South  America.     During  the  space  Gtfow  hours 
the  sky  was  illumined  with  thousands  of  shooting-stars, 
mingled  with  meteors  of  vast  magnitude.     This  phe- 
nomenon was  not  confined   to   Curnana,  but  extended 
from  Brazil  to  Greenland,  and  as  far  east  as  Weimar,  in 
Germany. 

On  the  13th  of  the  same  month,  in  1831.  a  meteoric 
ehower  occurred  at  Ohio,  and  also  near  Carthagena,  off 
the  coast  of  Spain.  At  the  latter  place,  luminous  meteors 
of  large  size  were  beheld,  one  of  which  left  behind  it  an 

Are  meteors  at  all  times  equally  abundant  ? 
What  two  great  epochs  exist  1 


METEORIC   SHOWERS.  221 

enormous  train,  tinted  with  prismatic  hues,  its  trace  con- 
tinuing visible  for  the  space  of  six  minutes.  On  the  same 
day  of  the  following  year,  vast  numbers  of  shooting  stars 
fell  at  Mocha  on  the  Red  Sea,  upon  the  Atlantic  ocean, 
and  in  Switzerland.  The  same  brilliant  spectacle  then 
appeared  in  various  parts  of  England ;  the  sky  being 
illumined  soon  after  midnight  by  the  rushing  of  thou- 
sands of  meteors  in  every  direction. 

542.  But  by  far  the  most  magnificent  display  of  this 
kind  occurred  on  the  night  of  the  12th  and  morning  of 
the  13th  of  November,  1833.     It  extended   from  the 
northern  lakes  to  the  south  of  Jamaica,  and  from  61° 
W.  Long,  in  the  Atlantic  to  about  150°  W.  Long,  on 
the  Pacific  ocean  near  the  equator.     For  the  space  of 
seven  hours,  from  9  P.  M.  to  4  A.  M.,  the  heavens  blazed 
with  an  incessant  discharge  of  fiery  meteors,  that  burst 
in  countless  numbers  from  the  cloudless  sky.     At  times 
they  appeared  as  thick  as  snow-flakes  falling  through 
the  air,  as  large  and  as  brilliant  as  the  stars  themselves  ; 
and  it  required  no  vivid  imagination  to  suppose,  that 
these  celestial  bodies  were  then  actually  rushing  towards 

he  earth. 

543.  VA  RIETIES.    The  luminous  bodies  of  this  shower 
seemed  to  be  divided  into  three  kinds.     The  first  con- 
sisted of  bright  lines  traced  through  the  sky,  as  if  by 
a  point.     The  second  of  fiery  balls,  that  occasionally 
darted  across  the  heavens,  trailing  behind  them  extend 
ed  and  luminous  trains,  which  generally  continued  visi- 
ble for  many  minutes.    The  third  of  radiant  bodies,  that 
continued  almost  immovable  for  a  considerable  time. 

544.  Meteors  of  the  first    class   occurred   in  great 
abundance.     At  Union  Town,  Pennsylvania,  they  were 
seen  shooting  along  like  streams  of  fire  with  the  rapid- 
ity of  lightning ;  often  crossing  half  the  visible  heavens 
in  less  than  a  second. 

At   New  York,  about  a  quarter  past  five  o'clock,  a 
meteor  of  the  second  class  was  beheld  rushing  from  the 

Describe  the  meteoric  showers  of  November,  1799, 1831,  1832  and  1833. 
In  the  shower  of  1833  how  many  kinds  of  meteors  were  noticed  1 
^Describe  them,  and  give  the  instances. 


222  LUMINOUS    PHENOMENA. 

zenith,  and  marking  its  course  by  a  fiery  line  apparently 
two  or  three  inches  wide.  After  passing-  downward  to 
a  considerable  distance,  it  formed  into  a  bait  of  the  appa- 
rent size  of  a  man's  hat,  and  then  returning  on  its  path, 
assumed  a  serpentine  figure.  It  lay  extended  through 
the  sky  for  the  space  of  several  minutes,  and  then  struck 
off  to  the  west. 

A  meteor  of  the  third  kind  was  visible  in  the  north- 
east, at  Poland,  Ohio,  for  more  than  an  hour.  It  first 
appeared  in  the  form  of  a  pruning  hook,  apparently 
twenty  feet  long,  and  eighteen  inches  broad,  and  shone 
with  great  splendor.  At  Niagara  Falls,  at  two  o'clock 
in  the  morning,  an  extended  luminous  body  like  a  square 
table  was  noticed  in  the  zenith.  It  remained  for  a  time 
nearly  stationary,  sending  out  on  every  side  broad  streams 
of  light. 

545.  It  was  distinctly  noticed  by  many  attentive  and 
accurate   observers,  that   all   the   meteors  appeared   to 
emanate  from  a  certain  region,  situated  in  the  constella- 
tion Leo  ;  and  that  during  the  whole  display  this  point 
was  stationary  among  the  stars  for  more  than   two 
hours;  thus  proving,  that  the  source  of  the  meteoric 
shower  was  beyond  the  atmosphere  of  the  earth  ;  for  had 
it  been  within,  it  must  have  moved  eastward,  in  the 
direction  of  the  earth's  daily  motion. 

546.  For  four  successive  years,  after  the  great  No- 
vember shower  of  1833,  an  unusual  number  of  meteors 
was  observed  in  America  at  this  period.     The  phenome- 
non ceased,  upon  this  continent,  in  1838;  but.  an  extra- 
ordinary display  then  occurred  at  Vienna,  more  than  a 
thousand  meteors  falling  in  the  course  of  six  hours. 

547.  AUGUST  EPOCH.     The  second  meteoric  period 
occurs  on  the  9th  and  10th  of  August.     It  was  first  dis- 
tinctly announced  in  1827  by  Thomas  Foster  of  London, 

What  fact  was  distinctly  noticed  by  attentive  observers  1 

Where  is  this  point  situated  1 

What  is  inferred  from  the  circumstance  that  it  was  stationary? 

For  how  many  years  after  1833  did  this  phenomenon  appear  ? 

When  does  the  second  meteoric  period  occur  ? 

By  whom  was  it  first  announced  1 


METEORIC    SHOWERS.  223 

in  his  Encyclopedia  of  Natural  Phenomena.  The  num- 
ber of  meteors  observed  at  this  epoch  is  probably  five  or 
six  times  more  than  the  usual  nightly  average,  which 
has  been  estimated  by  Mr.  E.  C.  Herrick,  of  New  Haven, 
at  not  more  than  thirty  per  hour  for  four  observers. 

548.  From  1836  to  the  present  year,  scarcely  a  season 
has  passed  without  an  unusual  display  of  meteors  at  thi° 
period,  in  some  quarter  of  the  globe. 

On  the  9th  of  August,  1839,  four  observers  at  New 
Haven  beheld  691  shooting-stars  in  the  course  of  fiv.°. 
hours,  a  third  part  surpassing  in  brightness  stars  of  the 
first  magnitude.  On  the  ensuing  night,  491  were  seen 
in  the  space  of  three  hours,  by  the  same  number  of  ob- 
servers ;  and  at  Vienna  in  Austria,  during  the  same 
evening,  shooting-stars  descended  at  the  rate  of  sixty 
per  hoar. 

Upon  the  annual  return  in  1842,  490  meteors  fell  at 
Parma  in  Italy,  and  779  at  Vienna.  Many  were  like- 
wise seen  at  Brussels.  At  New  Haven,  in  the  space 
of  fifty  minutes,  89  were  seen,  one  of  which  equaled 
Jupiter  in  splejidor. 

In  1847,  at  Manlius,  N.  Y.,  415  meteors  were  seen  on 
the  morning  of  the  llth  of  August  in  the  course  of  two 
hours,  commencing  at  midnight  and  ending  at  2  o'clock 
A.  M.  On  the  10th  of  August,  1848,  475  meteors  were 
noted  at  New  Haven,  in  the  space  of  two  hours  and  a 
half,  by  Mr.  E.  C.  Herrick  and  three  other  observers. 
Many  of  them  exceeded  in  brilliancy  stars  of  the  first 
magnitude.  In  France,  on  the  same  night,  414  shooting- 
stars  were  beheld  by  two  observers,  within  a  period  of 
three  hours  and  a  quarter. 

549.  Like  the  meteors  of  November,  those  of  August 
appear  also  to  radiate  from  a  small  space  in  the  heav- 
ens, which  has  been  referred,  by  all  observers,  to  the 
constellation  Perseus. 

Shooting-stars  have  likewise  been  found  to  be  more 

What  is  said  in  regard  to  the  recurrence  of  this  shower  ? 
State  facts. 

What  is  said  respecting  the  source  of  the  August  me'.eors  1 
Where  is  it  situated  ? 


224  LUMINOUS    PHENOMENA. 

than  usually  abundant  on  the  18th  of  October,  the  6lh 
and  7th  of  December,  the  2d  of  January,  the  20th  of 
April,  and  from  the  loth  to  the  20th  of  June. 

550.  ORIGIN.     Prof.  Olnisted,  who  was  the  first  to 
present  his  views  upon  the  extraordinary  phenomenon, 
which  occurred  on  the  12th  of  November,  1S33,  has  ar- 
rived at  the  following  conclusions  froni  a  very  extensive 
examination  of  facts. 

That  the  source  of  the  meteors  is  a  body  possibly  of 
great  extent,  composed  of  matter  exceedingly  rare  like 
ihe  tail  of  a  comet.  That  it  revolves  about  the  sun 
within  the  orbit  of  the  earth,  its  period  of  revolution  be- 
ing probably  a  little  less  time  than  a  year, 

That  in  consequence  of  its  proximity  on  the  night,  in 
question,  the  extreme  parts  of  the  body  were  detached 
and  drawn  towards  our  globe,  by  ihe  force  of  gravity. 

That  its  altitude  above  the  surface  of  the  earth,  at  its 
nearest  point,  was  about  2238  miles;  and  that  the  de- 
scending fragments  entered  the  atmosphere  with  a  velo- 
city ranging  from  about  fourteen  to  twenty  miles  per 
second. 

That  these  fragments  were  combustible,  and  in  conse- 
quence of  their  amazing  velocity,  the  air  was  so  power- 
fully compressed  before  them,  that  they  took  fire,  and 
were  consumed  before  reaching  the  earth. 

551.  This  last  conclusion  will  appear  by  no  means 
incredible,  when  the  following  considerations  are  taken 
into  view. 

By  suddenly  forcing  down  a  solid  piston  to  the  bottom 
of  a  cylinder,  in  which  it  moves  air  tight,  sufficient  heat 
can  be  evolved  to  ignite  tinder;  and  this  occurs,  when 
the  air  within  the  cylinder  is  compressed  to  one-fifth  of 
its  original  volume.  Upon  the  supposition,  that  the  de- 
scending fragments  compressed  the  rarefied  atmosphere 
at  the  height  of  35  miles  only  to  the  density  of  common 
air,  the  amount  of  heat  developed  would  be  46,080° 

What  is  said  of  other  periods  ? 
Detail  Prof.  Olmsted's  theory. 

What  is  said  respecting  the  amount  of  heat  developed  by  the  condcnsa 
fcoli  of  the  atmosphere  7 


CHALDNl's    THEORY.  225 

Fah.;  an  intensity  nearly  three  times  greater  than  the 
highest  temperature  of  a  glass-house  furnace,  which  is 
16,000°  Fah. 

552.  If  the  nebulous  body  revolves  about  the  sun  in 
a  period  somewhat  less  than  a  year,  it  tends  to  explain 
the  occurrence  of  shooting-stars  at  all  seasons  (since  the 
earth  and  the  nebulous  body  would  then  be  always  com- 
paratively near  each  other),  and  will  also  favor  the  ex- 
planation of  the  meteoric  showers  which  have  happened 
towards  the  end  of  April. 

553.  Prof.  Olmsted  has  been  led  to  suppose,  from  the 
whole  course  of  his  observations,  that  the  nebulous  body 
in  which  the  meteors  originated,  might  be  identical  with 
the  zodiacal  light.     In  a  late  article  published  by  M. 
Biot,  Ihis  distinguished  philosopher  also  maintains,  that 
meteoric  showers  are  occasioned  by  the  zodiacal  light 
coming  in  periodic  contact  with  the  atmosphere  of  the 
earth. 

It  is  not  regarded  by  Prof.  Olmsted  as  essential  to  the 
truth  of  his  theory,  that  a  shower  of  meteors  should 
occur  upon  the  13th  of  everu  November 

554.  In  order  to  account  for  shooting-stars  in  gen- 
eral, including  alike  their  ordinary  and  extraordinary 
displays,  and  embracing  the  several  epochs,  the  views 
of  Chaldni  have  been  adopted  by  Arago  and  other  emi- 
nent philosophers 

555.  CHALDNI'S  THEORY.     This  theory  consists  in 
supposing,  that,  besides  the  planets,  millions  of  small 
bodies  are  constantly  revolving  about  the  sun,  which 
become  ignited  when  they  enter  the  terrestrial  atmos- 
phere.    They  are  not  considered  to  be  uniformly  spread 
throughout  space;  but  in  some  regions  to  be  diffusely 
scattered,  and  in  others  grouped  together  in  vast  multi- 
tudes,  formjng  zones  or  rings  around  the  sun ;  many 
of  which  cross  the  path  of  the  earth. 

The  ordinary,  nightly  phenomenon  of  shooting-stars, 


If  the  nebulous  body  revolves  about  the  sun  in  a  little  less  time  than  a 
year,  what  does  it  tend  to  explain  ? 
What  is  M.  Biot's  opinion  7    What  is  Chaldni's  theory  1 


226  LUMINOUS    PHENOMENA. 

is  then  imagined  to  arise,  when  the  earth,  in  her  pro- 
gress through  the  heavens,  traverses  those  regions  which 
contain  only  a.  few  of  these  bodies  ;  but  when  the  zones 
are  encountered,  and  the  globe  passes  amid  countless 
numbers,  the  display  is  proportionally  greater,  and  the 
meteors  occasionally  descend  in  magnificent  showers. 
Amid  this  vast  collection  solid  masses  of  considerable 
size  are  supposed  to  exist,  and  should  one  of  these  enter 
the  atmosphere  of  the  earth,  a  meteorite  with  all  its 
splendors  sweeps  across  the  sky. 

Such  at  present  is  the  general  state  of  our  knowledge 
in  regard  to  shooting-stars. 


CHAPTER    1IT. 

OF  THE  AURORA  BOREALIS  OR  NORTHERN  LIGHT. 

556.  THE  Aurora  Borealis  is  a  luminous  appearance 
in  the  northern  sky,  whic^i  presents,  when  in  full  dis- 
play, a  spectacle  of  surpassing  splendor  and  beauty.     It 
has  in  all  ages  been  an  object  of  wonder  and  mystery, 
and  still  continues  so  ;  for  although  many  valuable  facts 
have  been  brought  to  light   by  the   investigations  of 
scienue,  the  cause  of  this  brilliant  phenomenon  is  yet  in- 
volved in  obscurity. 

557.  CONSTITUTION.     Notwithstanding  its  fantastic 
motions,  and    momentary  changes  in    brightness   and 
color,   the   aurora,  according  to  the  best  observations, 
still  preserves,  amid  all  its  fluctuations,  certain  invaria- 
ble characteristics  of  form  and  position.     It  consists  of 
a  dark  segment,  an  arch  of  Light,  luminous  streamers, 
and  a  corona  or  crown. 

558.  -DARK   SEGMENT.     All  observers  in  the  high 
latitudes  of  Europe,  agree  in  stating,  that  before  the 

What  does  chapter  third  treat  of  1 
What  is  the  Aurora  Borealis? 
Of  what  does  it  consist  1 
Describe  the  dark  segment. 


DARK    SEGMENT. 


227 


aurora  appears,  the  sky  in  the  northern  horizon  assumes 
a  darkish  hue,  which  gradually  deepens,  until  a  circular 
segment  is  formed,  bordered  by  an  arch  of  light,  extend- 
ing from  east  to  west.  The  segment  presents  the  ap- 
pearance of  a  cloud,  its  tint  is  light  in  the  lower  lati- 
tudes, and  grows  darker  as  we  advance  to  the  north,  up  to 
a  certain  limit ;  after  this  the  reverse  occurs,  and  when 
high  latitudes'  are  attained  it  becomes  so  faint  as  to  be 
scarcely  visible.  At  Upsal  and  Christiana  it  is  some- 
times black  or  of  a  deep  gray,  which  changes  into  a 
violet. 

During  a  splendid  aurora,  that  occurred  at  Toronto  in 
Dec.  1835,  and  which  is  described  by  Capt.  Bonnycastle, 
a  dark,  black  changing  mass,  was  visible  below  the  lu- 
minous arch,  (fig.  42,)  and  in  a  remarkable  phase  of  the 
aurora,  when  several  bright  bows  were  seen  at  once, 
the  interval  between  the  second  and  third  assumed  a 
blackness  of  the  deepest  intensity. 

Kg.  40. 


AURORA   SEEN   AT    TORONTO. 


559.  A  difference  of  opinion  exists  in  regard  to  tha 
nature  of  this  segment.     From  numerous  observations 
made  at  Dorpat  in  Russia,  Struve  infers,  that  the  dark- 
Is  it  real  or  imaginary  1 


228  LUMINOUS  "PHENOMENA. 

ness  is  simply  the  effect  of  contrast  with  the  luminous 
arch  ;  while,  from  equally  extensive  researches  at  Abo 
in  Finland,  Argelander  concludes,  that  the  segment  is 
something  real ;  since  the  portion  of  the  sky  it  occupies, 
is  darker  than  common,  before  the  bright  bow  of  the 
aurora  appears. 

560.  ARCH  OP  LIGHT.  The  dark  segment  is  bounded 
by  a  luminous  arch  or  bow,  varying  in  width  from  one  to 
three  apparent  diameters  of  the  moon.  Its  lower  edge 
is  clearly  denned,  but  the  upper  is  only  so  when  the 
arch  is  narrow,  for  as  the  width  increases,  it  gradually 
blends  with  the  brightness  of  the  sky.  The  color  of 
the  bow  is  a  pale  white,  which  becomes  more  pure  and 
brilliant  near  the  polar  regions. 

According  to  the  most  accurate  observations,  this  arch 
has  a  tendency  to  place  itself  at  right  angles  to  the 
magnetic  meridian,  or  in  other  words,  to  the  direction 
of  a  compass-needle  at  rest.  (G.  985.)  This  fact  was 
particularly  noticed  by  Lieutenant  Hood,  who  accom- 
panied Franklin  in  his  northern  expedition  in  1819. 

5b'l.  The  centre  of  the  auroral  arch  probably  coin- 
cides with  the  north  magnetic  pole  of  the  earth,  which 
is  situated  in  70"  N.  Lat.  In  our  own  country,  the  com- 
pass-needle points  to  the  north,  and  the  arch  crosses  the 
heavens  from  east  to  west ;  but  in  some  parts  of  Green- 
land, the  needle  is  directed  to  the  west,  and  the  arch  is 
then  seen  extending  from  north  to  south. 

In  the  year  1838,  when  Simpson  wintered  at  Fort 
Confidence,  in  66°  54'  N.  Lat.,  he  found  the  needle 
always  pointing  to  the  north-east,  and  the  auroral  arches 
invariably  spanning  the  heavens  at  right  angles,  from 
norLli-west  to  south-east. 

At  Melville  Isle,  in  74°  30'  N.  Lat.,  the  luminous 
arches  were  seen  by  Parry  in  the  south;  the  north 
magnetic  pole  of  the  earth  being  then  in  that  direction. 

502.  Tiiis  beautiful  bow  of  light  is  not  stationary, 

Describe  the  arch  of  light.    Its  color  and  position. 

What  is  its  position  in  some  parts  of  Greenland  1 

What  was  its  position  at  Fort  Confidence  and  at  Melville  Iflle  ? 

Is  the  arch  of  light  stationary  7 


ARCH    OF    LIGHT. 


229 


but  frequently  rises  and  falls;  and  when  the  aurora  ap- 
pears in  great  splendor,  several  arches  are  seen  at  the 
same  time  crossing  the  sky,  ascending  gradually  from 
the  horizon  to  the  zenith,  arid  passing  over  in  succession 
with  their  summits  moving  in  or  parallel  to  the  magnetic 
meridian ;  presenting  to  the  eye  broad  belts  of  light, 
increasing  in  brightness  as  they  approach  the  zenith. 

563.  No  less  than^ve  such  arches  were  seen  at  once 
by  Lieut.  Hood  ;  but  similar  phenomena,  of  far  greater 
beauty,  were  witnessed  by  M.  Lottin  at  Bossekop,  in 
West  Finmark,  during  the  winter  of  1838-9.  (Figs. 
43,  44.) 

Fig.  43. 


AURORA  SEEN   AT   BOSSEKOP. 

Fig-  44. 


AURORA   SEEN   AT   BOSSEKOP. 


What  phenomena  were  beheld  Vy  Lieut.  Hood  and  M.  Lottin '» 


230  LUMINOUS    PHENOMENA. 

On  one  occasion,  as  many  as  nine  auroral  arches 
were  visible,  separated  by  distinct  intervals,  and  in  their 
arrangement  resembling  magnificent  curtains  of  light, 
hung  one  behind  and  below  the  other,  their  dazzling 
folds  extending  completely  across  the  sky. 

564.  STREAMERS.    Although  the  luminous  arch  pre- 
serves, in  the  main,  its  curved  form,  it  is  subject  to  con- 
stant changes.    Now  at  one  extremity,  now  at  the  other, 
and  again  at  intermediate  points,  a  cloud  of  light  will 
break  suddenly  forth,  separating  into  rays  which  stream 
upward  like  tongues  of  fire,  moving  at  the  same  time 
backwards  and  forwards,  along  the  auroral  bow. 

The  origin  of  the  streamers  is  in  the  luminous  arch, 
from  which  they  rise  in  the  form  of  tapering  rays  or 
pencils  of  light,  ever  in  motion,  and  continually  varying 
in  brilliancy,  number,  magnitude,  and  color.  At  one 
moment,  a  ray  is  just  visible  above  the  arch,  faintly 
glowing  in  the  sky;  at  the  next  it  is  seen  shooting  up- 
ward in  a  pyramid  of  flame  and  at  the  same  time  moving 
majestically  across  the  heavens.  As  suddenly  its  bright- 
ness fades,  and  as  quickly  it  is  again  beheld,  flashing 
forth  with  renewed  splendor. 

565.  COLOR.     During  the  extraordinary  displays  of 
the  aurora  in  our  own  latitude,  the  sky  is  frequently 
seen   suffused  with  a  flush    of  rosy   light,  while    the 
streamers  assume  a  crimson  hue.     In  that  which  oc- 
curred on  the  night  of  the  14th  of  November.  1837, 
the  upper  extremities  of  the  streamers  were  of  the  deep- 
est scarlet,  while  below  they  were  brilliantly  white.    But 
the  richest  tints  appear  in  the  arctic  regions.     In  the 
auroras  witnessed  at  Bossekop,  the  rays,  at  their  base, 
glowed  with  a  blood-red  hue,  the  middle  was  of  an  em- 
urald  green,  and  the  rest  of  a  pure  transparent  yellow. 
During  a  brilliant  display  that  occurred  at  yort  Con- 
fidence, on  the  5th  of  March,  1839,  the  rays  were  tinged 
with  red,  purple,  and  green. 

566.  CORONA  OR  CROWN.     The  vivid  rays  that  dart 


What  is  said  in  regard  to  the  streamers,  their  origin  and  color '{ 


CORONA.  231 

forth  from  the  luminous  arch  not  unfiequently  unite  at 
a  point  near  the  zenith ;  forming  a  brilliant  mass  of 
light  which  is  called  the  corona  or  crown.  The  aurora 
then  appears  in  its  greatest  splendor ;  the  sky  resembles 
a  fie"ry  dome,  and  over  the  streamers,  which  seem  like 
pillars  of  variegated  flame  supporting  the  corona,  radiant 
waves  and  flashes  of  light  pass  in  quick  succession.  The 
luminous  columns  at  this  time  are  apparently  shaken 
and  wave  with  a  tremulous  motion  ;  whence  they  have 
received,  under  these  circumstances,  the  name  of  merry 
dancers. 

At  Bossekop,  this  radiant  wave  was  seen  by  Lottin, 
crossing  and  re-crossing  with  rapid  undulations  the 
whole  broad  field  of  auroral  light.  These  coruscations 
are  generally  attended  with  color. 

567.  When,  in  the  northern  hemisphere,  a  needle  is 
delicately  balanced  upon  a  horizontal  axis,  its  north 
end  immediately  dips  downward  upon  its  being  magnet- 
ized. Such  an  instrument  is  called  the  dipping-needle. 
(C.  998.)  The  streamers  of  the  aurora  assume  the  same 
direction  as  the  dipping-needle,  and  are  parallel  to  each 
other ;  hence  the  corona  is  not  formed  by  any  actual 
union  of  the  streamers  near  the  zenith.  It  arises  from 
an  optical  illusion.  When  we  look  acfoss  an  extensive 
field  of  corn,  the  rows,  at  their  remote  ends,  seem  to  ap- 
proach each  other,  as  if  converging  to  a  point ;  though 
we  know  that  they  are  three  or  four  feet  apart,  through- 
out their  whole  distance.  In  like  manner  when  we 
gaze  at  the  auroral  streamers  with  their  bases  at  the 
horizon  and  their  summits  at  the  zenith,  they  will  in  like 
manner  apparently  converge  to  one  point,  forming  the 
corona,  whose  centre  is  in  the  line  of  the  dipping-needle. 
568.  Within  the  dark  segment  streamers  of  the  same 
color  are  frequently  seen,  rising  and  falling  like  columns 
of  smoke,  changing  their  hue  in  a  moment,  and  possess- 
ing all  the  motions  of  the  luminous  rays.  Like  the 

Describe  the  corona. 

When  does  it  appear  1 

How  is  it  formed  1 

What  is  observed  within  the  dark  segment  ? 


232  LUMINOUS    PHENOMENA. 

latter,  their  line  of  direction  is  parallel  to  that  of  the 
dipping-needle. 

569.  EXTENT.  The  aurora  is  not  a  local  appearance, 
for  it  is  beheld  simultaneously  in  places  widely  separa- 
ted from  each  other.     Thus,   on   the  5th  of  January, 
]769,  the  same  aurora  was  seen  in  France  and  Pennsyl- 
vania ;   and   a  magnificent  display  occurred  on  the  7th 
of  January,  1831,  which  was  visible  at  Lake  Erie,  and 
throughout   northern    and    central    Europe.      Another 
aurora,  that  happened   on  the  3d  of  September,  1839, 
was  seen  at  the'  Isle  of  Sky,  57°  22'  N.  Lat.,  at  Paris, 
New  Haven,  and  at  New  Orleans. 

570.  The  beautiful  phenomenon  of  the  northern  light 
is  not  confined  to  the  northern  hemisphere.    An  aurora 
an  sir  a  Us,  or  southern  light,  was  observed  by  Don  Ulloa, 
at  Cape  Horn,  in  1745  ;  and  various  displays  were  seen 
hy  Capt.  Cook,   in  the  high  southern  latitudes,  at  the 
same  time  that  the  northern  lights  were  visible  in  Eu- 
rope. 

In  the  late  Exploring  Expedition,  during  the  southern 
cruise  of  the  Peacock  and  Flying  Fish,  several  brilliant 
auroras  were  seen,  which  are  thus  recorded.  On 
the  18th  of  March,  1839,  there  was  "a  beautiful  display 
of  the  aurora  afistralis,  extending  from  S.  S.  W.  to  the 
east ;  the  rays  were  of  many  colors,  radiating  towards 
the  zenith  and  reaching  an  altitude  of  30°.  On  the 
19th,  in.  about  68°  S.  Lat.,  another  display  was  wit- 
nessed which  exhibited  a  peculiar  effect.  In  the  south- 
ern quarter  of  the  heavens  there  was  the  appearance  of 
a  dense  cloud,  resembling  a  shadow  cast  upon  the  sky, 
and  forming  an  arch  about  10°  in  altitude.  Above  this 
were  seen  coruscations  of  light,  rendering  all  objects 
around  the  ship  visible.  From  behind  this  cloud,  diverg- 
ing rays  frequently  shot  up  to  an  altitude  of  from  25°  to 
45°.  These  appearances  continued  until  the  day 
dawned." 


What  is  said  of  the  extent  of  the  aurora? 
Are  auroras  seen  in  the  southern  hemisphere  ? 
What  are  they  called? 
Give  instances. 


HEIGHT.  233 

571.  HEIGHT.     The  height  of  the  aurora  has  been 
variously  estimated      The  earlier  philosophers  computed 
its  altitude  at  several  hundred  miles ;  but  a  much  lower 
limit  is  assigned  by  later  observers.     An  aurora  which 
appeared  in  March,  1826,  at  different  places  in  England, 
was  calculated  by  Dr.   Dalton  to  be  100  miles  high. 
Observations  for  determining  the  elevation  of  the  splen- 
did aurora  of  January  7th,  1831,  were  made  by  Christie 
and  Hansteen,  but  their  computed  heights  varied  from 
23  miles  to  120. 

In  the  brilliant  display  that  happened  on  the  14th 
of  November,  1837,  the  estimated  altitudes  were  even 
more  discrepant,  varying  from  one  to  two  hundred 
miles. 

A  very  distinct  auroral  arch  was  seen  at  various  places 
throughout  the  Northern  arid  Middle  States,  at  about 
ten  o'clock  on  the  night  of  the  7th  of  April,  1847. 
From  the  observations  taken  by  Mr.  E.  C.  Herrick,  at 
New  Haven,  Ct.,  and  Dr.  P.  W.  Ellsworth,  at  Hartford, 
Ct.,  the  height  was  computed  by  the  former  gentleman, 
and  found  to  be  one  hundred  and  ten  miles.  These 
observations  having  been  made  under  favorable  circum- 
stances, and  being  accordant  with  each  other,  this  re- 
sult is  entitled  to  great  confidence. 

The  height  of  the  northern  lights  is  obtained  in  the 
way  that  has  been  already  described  ;  but  such  is  their 
fitful  nature  and  varying  form,  that  two  distant,  observers 
can  scarcely  ever  be  sure  that  they  have  measured  the" 
angular  height  of  the  same  part  of  the  aurora.  Hence 
arise  these  discordant  calculations  upon  the  same  phe 
nomenon. 

572.  There    is   every  reason  for  believing,  that  the 
auroral  light  is  at  times  very  near  the  earth,  and  even 
within  the  region  of  the  clouds. 

During  the  polar  expedition  of  Franklin,  in  1820,  ob- 
servations were  taken  by  Hood  arid  Richardson,  upon 
thiee  auroras,  at  stations  eighteen  leagues  distant  from 
each  other,  and  the  heights  which  they  obtained,  were 
found  to  vary  from  six  to  seven  miles;  while  an  aurora 

Relate  in  full  the  calculations  respecting  the  height  of  the  northern  light*. 


K34  LUMINOUS    PHENOMENA. 

beheld  by  Farquharson,  of  Scotland,  was  computed  to  be 
as  low  as  4300  feet.  Franklin  thus  remarks  :  "  The  fact 
that  the  aurora  exists  at  a  less  height  than  that  of  dense 
clouds,  was  evinced  at  Fort  Enterprise,  on  two  or  three 
occasions,  during  the  night  of  the  13th  of  February,  1821, 
and  particularly  about  midnight,  when  a  brilliant  mass 
of  light,  variegated  with  the  prismatic  colors,  passed 
between  a  uniformly  steady,  dense  cloud  and  the  earth. 
In  its  progress,  that  portion  of  the  cloud  which  the 
stream  of  light  covered  was  completely  concealed  until 
the  coruscation  had  passed  over  it,  when  it  appeared  as 
before." 

573.  A  similar,  but  more  extraordinary  phenomenon, 
which  occurred  during  his  third  Arctic  voyage,  is  thus 
related  by  Capt.  Parry.     "  While  Lieutenants  Sherer, 
Ross,  and  myself  were  admiring  the  extreme  beauty  of 
the  northern  lights,  we   all  simultaneously  uttered  an 
exclamation  of  surprise,  at  seeing  a  bright  ray  of  the 
aurora  shoot  suddenly  downward  from  the  general  mass 
of  light,  and  between  us  and  the  land,  which  was  there 
distant  only  three  thousand  yards.     I   have  no  doubt, 
that  the  ray  of  light  actually  passed  within  that  dis 
tance  of  us." 

574.  SOUNDS    ATTENDING    THE  AURORA.      It    has 
been  asserted,  that  the  aurora  is  sometimes  accompanied 
by  a  noise  like  the  rustling  of  silk,  or  the  sound  of  a 
fire  when  excited  by  the  wind  ;  but  much  difference  of 
opinion  has   arisen  upon  this  point.     Those  who  are 
incredulous  in  this  particular,  affirm  that  the  noise  in 
question  may  be  nothing  more  than  the  murmur  of  the 
ocean,  or  of  the  forest ;  the  rustling  of  the  snow  as  it  is 
driven  by  the  wind,  or  the  crackling  sound   that  arises 
from  its  freezing  ;  all  which,  it   is  said,  might  be  easily 
attributed  to  the  aurora,  when  the   mind   is  excited  by 
the  wondrous  spectacle,  and  susceptible  to  every  illusion 
• — the  splendors  that  burst  upon  the  sight,  and  the  sounds 
which  strike  the  ear  being  then   referred   to  the  same 
origin. 

Slate  the  facts  showing  that  the  aurora  is  at  times  very  near  the  earth. 
Give  the  facts  respecting  the  sounds  attending  the  aurora. 


AURORAL    SOUNDS.  235 

575.  Scoresby,  Richardson,  Franklin,  Parry  and  Hood, 
during  their  Polar  expeditions,  never  heard  any  sound 
which  they  considered  as  proceeding  undeniably  from 
the  northern  lights,  though  hissing  noises  were  heard 
during  the  auroral  displays  which  were  attributed  by 
them  to  one  or  more  of  the  preceding  causes.     These 
observers  do  not,  however,  deny,  that  at  times  audible 
sounds  proceed  from  the  aurora,  and  even  express  such 
a  belief,  founded  upon  the  concurrent  testimony  of  the 
natives  of  the  arctic  climes. 

576.  Credible    observers    in   Iceland,     Siberia,    and 
Scandinavia,  have  never  heard  these  singular  sounds ; 
nor  were  they  perceived   by  the   French  scientific  expe- 
dition, which  wintered  at  Bossekop,  in    1838-39  ;    but 
Hansteen  claims  to  have  established  their  existence  from 
a  series  of  observations  in  the  high  northern  latitudes. 
Upon  this  subject,  Simpson  thus  remarks  in  his  North- 
ern Discoveries  when  speaking  of  a  brilliant  aurora  seen 
by  his  attendant,  at  Fort  Confidence,  on   the  5th   of 
March,  1839,  "The  aurora  seemed  to  ascend  and  de- 
scend, accompanied  by  an  audible  sound  resembling  the 
rustling  of  silk.     This  lasted  about  ten  minutes,  when 
the  whole  phenomenon  suddenly  rose  upwards,  and  its 
splendor  was  gone.     Hitch  is  an  intelligent  and  credible 
person,  and  on  questioning  him  closely,  he  assured  me 
that  he  had   perfectly  distinguished  the  sound  of  the 
aurora  from  that  produced  by  the  freezing  of  the  breath, 
for  the  temperature  was  forty-four  degrees  below  zero. 
I  can  therefore  no  longer  entertain  any  doubt  of  a  fact 
uniformly  asserted  by  the  natives,  and  insisted  on  by 
my  friend   Mr.  Dease,  and  by  many  of  the  oldest  resi- 
dents of  the  fur  countries,  though  I  have  not  had  the 
good  fortune  to  hear  it  myself." 

V  577.  TIME.  The  appearance  of  the  northern  lights 
is  not  confined  to  any  particular  hour  of  the  night,  a 
fact  which  is  fully  proved  by  the  circumstance  that  the 
same  display  is  frequently  witnessed  at  places  widely 
differing  in  longitude.  Thus,  if  the  aurora  extends 


Doea  the  aurora  appear  at  any  particular  hour  ? 


236  LUMINOUS    PHENOMENA. 

from  Boston,  Mass.,  to  Berlin,  in  Germany,  and  is  bi> 
held  simultaneously  at  these  cities,  the  difference  in  the 
reckoning  of  time  will  be  n early  five  hours  and  a  half 
(C.  939). 

578.  There  is  much  reason  for  believing  that  the  aurora 
sometimes  occurs  during  the  day,  though  rendered  invis- 
ible by  the  presence  of  the  sun.     Richardson  perceived 
at  Bear  Lake,  the  motion  of  the  aurora  before  the  entire 
disappearance  of  twilight,  and  even  during  the  day  he 
discerned  clouds,  arranged  in  columns  and  arches,  resem- 
bling those  of  the  northern  lights.     Besides,  as  we  shall 
show  hereafter,  a  brilliant  display  of  this  phenomenon 
is  always  accompanied  by  a  greater  or  less  disturbance 
of  the  magnetic-needle,  (C.  997,)  and  as  these  disturb- 
ances take  place  in  the  day  as  well  as  in  the  night,  it  is 
reasonable  'to  infer  that  they  are  caused  by  the  presence 
of  an  invisible  aurora. 

579.  FREQUENCY.     This  phenomenon  is  more  fre- 
quently seen  in  winter  than  in  summer ;  we  must  not, 
however,  hastily  conclude  from  this  circumstance,  that 
the  number  of  auroras  during  the  former  season  is  actu- 
ally greater,  for  the  increased  length  of  the  nights  du- 
ring the  winter  would  enable  us  then  to  see  more  dis- 
plays of  the  northern  light,  even  if  the  times  of  its  occur- 
rence   were    equally  distributed   throughout  the   year. 
About  the  period  of  the  equinoxes  they  also  appear  to  be 
more  frequent.     These  facts  are  shown  from  the  follow- 
ing table  of  Kaemtz,  which  gives  the  number  of  auroras 
that  have  been  seen  in  each  month. 

NUMBER   OF  AUROHA   BOREALES   IN  EACH   MONTH. 

January,  229.  July,              87. 

February,  307.  August,        217. 

March,  440.  September,  405. 

April,  312.  October,      497. 

May,  184.  November,  285. 

June,  65.  December,  225. 


Why  is  it  supposed  sometimes  to  occur  ID  the  day  1 
What  is  said  respecting  the  frequency  of  its  appearance  in  -winter  and 
ummer?    Recite  the  table. 


FREQUENCY.  237 

580.  In  addition  to  this  annual  variation,  there  ap- 
pears to  be  another  which  extends  through  a  consider- 
able number  of  years,  but  of  which  very  little  is  known. 
Thus,  from  1707  to  1752,  the  northern  lights  became 
more  and  more  frequent ;  but  after  the  latter  date,  a 
period  of  twenty  years  occurred,  in  which  they  dimin- 
ished in  number. 

.  An  increase  in  their  frequency  began  in  1820,  and 
since  that  period  many  magnificent  displays  have  been 
witnessed. 

The  number  observed  for  the  last  ten  years,  at  New 
Haven,  Ct,  by  Mr.  E.  C.  Herrick,  is  shown  in  the  fol- 
lowing table. 


From  May 

,  1838,  to  May,  1839,  "'  ""ST* 

u 

1839,    "   1840, 

36. 

tt 

1840,    «   1841, 

36. 

it 

1841, 

1842, 

21. 

u 

1842, 

1843, 

7. 

u 

1843, 

1844, 

7. 

u 

1844, 

1845, 

12. 

tt 

1845, 

1846, 

19. 

tt 

1846,    "   1847, 

20. 

u 

1847,    "   1848, 

28. 

Between  the  12th  of  September,  1838,  and  the  18th 
of  April,  1839,  no  less  than  one  hundred  and  forty-three 
distinct  auroras  were  seen  by  the  French  observers  at 
Bossekop.  They  were  most  frequent  at  the  period 
when  the  sun  was  below  the  horizon,  viz. :  from  the 
17th  of  November  to  the  25th  of  January.  During  thia 
night  of  ten  weeks,  sixty-four  auroras  were  visible. 

581.  DISTURBANCE  OF  THE  MAGNETIC-NEEDLE. 
During  the  prevalence  of  the  aurora,  the  compass-needle, 
instead  of  remaining  motionless,  in  the  magnetic  meridi- 
an, is  often  much  disturbed.  Sometimes  it  is  dejlectcd 
toward  the  east  several  minutes  and  even  degrees  ;  then 


Is  there  any  other  probable  variation  1 

Recite  the  table. 

What  is  said  respecting  the  disturbance  of  the  compaa»-needle  1 


238  LUMINOUS    PHENOMENA. 

it  is  agitated,  and  returns  either  slowly  or  rapidly,  to 
the  meridian,  which  it  passes  at  times  and  moves 
toward  the  west.  These  deviations  are  as  changeable 
as  the  phenomenon  itself.  When  the  arch  is  motionless 
the  needle  is  quiet ;  its  disturbance  commences  when  the 
streamers  begin  to  play. 

582.  Franklin  observed  at  Fort  Enterprise,  that  the 
disturbance  of  the  needle  was  simultaneous  with  some 
change  in  the  form  or  action  of  the  northern  lights,  and 
that  after  being  deflected  it  returned  to  its  former  posi- 
tion very  gradually,  not  resuming  it  before  the  follow- 
ing morning,  and    sometimes    even   not   before   noon. 
Moreover  when  the  auroral  arch  was  either  at  right 
angles  to  the  meridian,  or  its  western  extremity  north 
of  west,  the  needle  was  defected  toward  the  west ;   but 
if  its  western  extremity  was  south  of  west,  the  needle 
moved  toward  the  east. 

During  the  aurora  of  November  14th,  1837,  the  en 
tire  range  of  the  needle  at  New  Haven,  was  observed  by 
Messrs.  Herrick  and  Haile  to  be  nearly  six  degrees.  It 
was  not  until  the  morning  of  the  next  day,  between 
seven  and  nine  o'clock,  that  the  needle  was  at  rest  in 
its  usual  position. 

583.  This  effect  upon  the  magnetic  needle  during  the 
prevalence  of  the  northern  lights,  was  noticed  for  the 
first  time  by  Celsius  and  Hiorter,  at  Upsal,  on  the  1st 
of  March,  1741. 

584.  It  is  asserted  by  Wilke,  that  when  the  aurora 
appears  in  great  splendor,  the  position  of  the  dipping- 
needle  is  as  variable  as  that  of  the  compass-needle  ;  the 
former  rising  and  falling  with  the  northern  crown. 

Hansteen  has  also  observed,  that  the  dipping-needle 
descends  very  much  below  ifcs  usual  position  before  the 
aurora  is  visible  ;  but  that  after  the  display  commences 
it  begins  to  rise :  and  more  rapidly  in  proportion  to  ita 
brightness.  The  needle  then  slowly  resumes  its  origi- 
nal position,  which  it  frequently  does  not  attain  unuj 


How  great  was  its  range  at  New  Haven,  November  14th,  18371 
What  has  been  observed  respecting  the  dipping-needle  1 


CAUSE.  239 

twenty-four  hours  have  elapsed.  From  numerous  ob- 
servations at  Bossekop,  M.  Bravais  has  likewise  obtained 
the  same  results. 

585.  CAUSE.     No  satisfactory  explanation  has  ever 
been  given  of  this  singular  phenomenon  :  that  a  connec- 
tion exists  between  the  aurora  and  the  magnetism  of 
the  earth,  is  evident  from  the  preceding  facts  ;  but  the 
nature  of  that  connection  is  still  unknown. 

To  trace  all  the  hypotheses  which  have  been  started 
would  be  an  unprofitable  task ;  but  a  glance  at  some  of 
the  most  prominent  may  be  given.  Canton  supposes 
the  aurora  to  be  caused  by  the  passage  of  electricity 
from  positive  to  negative  clouds,  in  the  upper  and  rarefied 
regions  of  the  atmosphere.  He  adduces  in  support  of 
this  view  the  fact,  that  when  the  air  within  a  long,  glass 
tube  is  rarefied,  and  electricity  passed  through  it,  the 
•whole  tube  is  illumined  by  flashes  of  light  traversing 
its  entire  length.  It  may,  however,  be  stated  in  reply, 
that  the  general  height  of  the  northern  lights  far  exceeds 
that  of  the  highest  clouds. 

586.  Beccaria  supposes,  that  there  is  a  constant  cir- 
culation of  the  electric  fluid  from  north  to  south,  and 
that  the  aurora  is  seen,  whenever  the  electrical  current 
passes  nearer  than  usual  to  the  earth,  or  the  state  of  the 
atmosphere  is  such  as  tn  vender  it  luminous.     Faraday 
has  demonstrated,  that  the  electricity  of  the  earth  neces- 
sarily tends  from  the  equator  towards  the  poles ;  and 
has  suggested,  that  the  aurora  may  possibly  arise  from 
an  upward  current  in  the  atmosphere  flowing  back  from 
the  poles  towards  the  equator. 

Kaemtz  conjectures,  that  since  a  spark  is  perceived 
every  time  an  electric  current  produced  by  a  magnet  is 
broken,  the  northern  lights  may  perhaps  be  caused  by  a 
rupture  in  the  magnetic  equilibrium  of  the  globe.  At  the 
same  time,  however,  he  utterly  disclaims  the  idea  of  ac- 
counting for  all  the  circumstances  of  this  wonderful  phe- 
nomenon, in  our  present  imperfect  state  of  knowledge. 


What  is  known  of  the  origin  of  the  northern  lights  ? 
State  the  hypotheses  given. 


240  LUMINOUS    PHENOMENA. 

587.  UTILITY.  The  light  of  the  aurora,  from  its  fre- 
quency and  splendor,  serves  materially  to  relieve  the 
darkness  and  enliven  the  gloom  of  the  long  polar  night. 
During  this  period,  its  play  is  almost  incessant,  (Art.  580,) 
and  its  coruscations  exceedingly  vivid  and  beautiful. 

So  brilliant  is  the  aurora  in  these  regions,  that  Mau- 
pertius  arid  others,  who  were  sent  to  Lapland  in  1735, 
by  the  Academy  of  Sciences  of  Paris,  for  the  purpose  of 
measuring  an  arc  of  the  meridian,  were  enabled  to  pur- 
sue their  difficult  work  by  the  light  it  afforded,  long 
after  the  sun  had  ceased  to  be  visible.  And  Maupertius 
remarks,  that  its  light,  together  with  that  of  the  moon 
and  stars,  is  sufficient,  during  this  season,  for  most  of 
the  occasions  of  life. 


What  useful  purpose  doce  the  aurora  subserve  in  the  polar  regional 


PART    VII. 

MISCELLANEOUS    PHENOMENA. 


CHAPTER  I. 

DF  THE  FALL  OF  TERRESTRIAL  SUBSTANCES  FOREIGN  TO  THE 
ATMOSPHERE. 

588.  IN  addition  to  storms  of  rain,  hail,  and  snow, 
which  are  products  peculiar  to  the  atmosphere,  and  are 
the  results  of  the  operations  of  well-known  agencies  and 
laws,  showers  of  matter  of  a  terrestrial  nature  have  not 
(infrequently  occurred,  which  have  been  traced,  upon  close 
examination,  to  a  mineral,  vegetable,  and  even  animal 
origin.     The  most  remarkable  of  these  singular  phenomena 
are  dust-storms  and  Hood-rains,  which  will  now  be  de- 
scribed. 

DUST-STORMS  AND  BLOOD-RAINS. 

589.  From  time  to  time,  and  in  regions  of  the  globe 
widely  separated  from  each  other,  dust  in  large  quantities 
has  descended  from  the  heights  of  the  atmosphere,  not  only 
upon  the  land,  but  also  far  out  on  the  ocean,  hundreds 
of  miles  from  the  shore.     It  is  entirely  distinct  from  that 
which  is  disseminated  through  the  air  by  the  winds,  during 
the  eruption  of  volcanoes,  and  for  many  years  has  been 
described,  by  observers   and  writers,  under   the   various 
names  of  dust-storms,  dust-rain,  red  fogs,  Sirocco  dust, 

What  is  the  subject  of  part  seventh  1 

Of  what  does  this  chapter  treat  1 

la  addition  to  storms  of  rain,  ha.il,  and  snow,  what  other  kinds  of 
showers  have  not  unfrequently  happened  1 

What  are  the  most  remarkable  of  these  phenomena  1 

What  is  said  respecting  the/aM  of  dust  from  the  heights  of  the  atmo- 
sphere ! 

What  are  the  various  names  under  which  this  phenomenon  has  been 
described  ! 

11 


242          MISCELLANEOUS  PHENOMENA. 

African  dust,  sea-dust,  Atlantic  dust,  and  tradewind- 


"590.  This  dust  not  only  falls  dry,  in  the  form  of  a  fine, 
impalpable  powder,  but  is  occasionally  mingled  with  rain, 
hail,  and  snow,  which  it  dyes  with  its  own  hue.  As  it 
is  usually  of  a  reddish  color,  these  showers  of  rain  and 
storms  of  hail  and  snow  have  received  the  appellation  of 
Hood-rains. 

DU8T-STOHMS. 

591.  INSTANCES.     On  the  20th  of  October,  1755,  a 
Hack  dust,  like  lamp-black,  fell  in  Shetland,   between 
3  and  4  o'clock  in  the  afternoon.     The  sky  at  the  time 
was  hazy,  and  the  dust  fell  in  such  quantities  as  to  cover 
the  hands -and  faces  of  persons  exposed  to  it,  and  to  black- 
en their  linen. 

592.  During  the  5th  and  6th  of  March,  1803,  while 
the  wind  was  blowing  from  the  south-east,  a  shower  of 
red  dust  fell  in  Italy.     Ten  years  afterwards,  on  the  14th 
of  March,  1813,  a  similar  storm  occurred  at  the  town  of 
Gerace,  in  Calabria.     According  to  Prof.  Sementini,  of 
Naples,  the  wind,  in  the  early  part  of  the  day,  blew  from 
a  western  quarter,  bringing  up  dark,  heavy  clouds   from 
the  sea  over  the  land. 

At  about  2  o'clock  in  the  afternoon  the  wind  sub- 
sided, while  a  deep  gloom  pervaded  the  air,  and  the  clouds 
'grew  red  and  threatening.  Thunder  followed,  and  soon 
after  red  dust,  mingled  with  red  ram  and  sri/s>io,  descend- 
ed upon  the  town.  This  dust  had  the  appearance  of  a 
jme  powder. 

593.  A  shower  of  dust  fell  at  Malta  on  the  15th  of 
May,  1830,  and  at  the  same  time  a  similar  fall  occurred 
in  the  bay  of  Palmas,  in  Sardinia,  while  a  Sirocco  wind 
was  blowing  from  a  south-easterly  quarter.     The  Maltese 
dust  was  of  a  brownish-red  hue.     Some  of  it  wao  collected 
by  Mr.  R.  G.  Didman,  of  "the"  ship   Revenue,  atid  for- 


"What  are  blood-rains,  and  why  are  they  so  called  ? 
Relate  the  various  instances  given  of  dust-storms  in  Shetland.  It;Jy. 
Gerace,  Malta,  and  Genoa. 


DUST-STORMS.  248 

warded  to  Mr.  Charles  Darwin,  an  eminent  English  nat- 
uralist, for  examination. 

594.  On  the  16th  of  May,  1846,  a  shower  of  Sirocco- 
dust  occurred  at  Genoa,  having  the  same  brownish-red 
hue  as  the  dust  which  fell  at  Malta  in  1830. 

Six  months  afterwards  a  remarkable  storm  of  this  nature 
swept  over  Lyons,  in  France,  and  so  thickly  did  the  dust 
descend,  that  the  amount  which  fell  at  this  time  was  com- 
puted to  weigh  no  less  than  thirty-six  tons. 

595.  In  the  year  1831,  the   ship  Beagle,  under  the 
command   of   Captain   Fitzroy,  was   dispatched   by   the 
British  government  on  a  voyage  of  scientific  discovery 
around  the  world.     Mr.  Darwin,  the  naturalist  just  men- 
tioned, accompanied  the  expedition,  and  during  the  voyage 
observed  a  dust-shower,  near  St.  Jago,  the  chief  of  the 
Cape  de  Verd  isles. 

The  morning  before  the  Beagle  anchored  at  Port  Praya, 
in  St.  Jago,  Mr.  Darwin  collected  a  little  package  of  im- 
palpable brown-colored  dust,  which  appeared  to  have 
been  filtered  from  the  wind  by  the  gauze  of  the  vane  at 
the  mast-head.  In  speaking  of  this  phenomenon,  he  re- 
marks, that  the  atmosphere  in  this  region  is  usually  filled 
with  a  haze,  caused  by  the  falling  of  this  fine,  brown- 
colored  dust.  By  the  kindness  of  a  friend,  Mr.  Darwin 
received  four  parcels  of  dust  which  fell  upon  the  deck 
of  a  vessel,  a  few  hundred  miles  north  of  the  Cape  de 
Verd  isles. 

596.  Much  valuable  information  respecting  dust-show- 
ers on  the  ocean  has  been  gathered  by  this  gentleman, 
who  has  found  fifteen  different  accounts  of  the  descent  of 
dust  upon  ships  when  far  out  on  the  Atlantic.     It  has 
often  fallen  upon  them  when  they  were  several  hundred, 
and  even  a  thousand  miles  from  the  coast  of  Africa,  and 
at  points  sixteen  hundred  miles  distant  in  a  north  and 
south  direction. 

597.  In  some  of  the  dust  collected  upon  a  vessel  three 
hundred  miles  from  land,  particles  of  stone  were  discov- 

State  what  is  suid  respecting  the  fall  of  dnst  on  the  ship  Beagle. 
What  is  known,  from  the  researches  of  Mr.  Danviu,  in  regard  to  tin 
Atlantic  dust  1 


244          MISCELLANEOUS  PHENOMENA. 

ered,  more  than  the  thousandth  of  an  inch  square,  mixed 
with  finer  matter.  It  falls  in  such  quantities  as  to  soil 
every  thing  upon  which  it  descends,  and  to  irritate  the 
eyes  of  persons  exposed  to  it.  Ships  have  even  been 
known  to  run  ashore,  owing  to  the  obscurity  of  the  atmo- 
sphere resulting  from  the  presence  of  this  dust. 

598.  The  occurrence  of  dust-showers  in  the  vicinity 
of  the  Cape  de  Verd  isles  has  been  noticed,  at  intervals, 
from  the  year  1579  to  the  pfesent  time.     The  extent  of 
the  region  over  which  they  here  prevail  varies,  according 
to  Darwin,  from  960,000  to  1,280,000  square  miles ;  bat 
a  greater  estimate  is  given  by  Captain  Tuckey,  who  sup- 
poses that  it  ranges  from  1,648,000  to  1,854,000  square 
miles.     The  Atlantic  dust  is  believed  by  Mr.  Darwin  to 
come  from  Africa,  since  not  only  does  wind  blow  from 
that  quarter  whenever  it  falls,  but  the  showers  also  occur 
during  those  months  when   the  harmattan  is  known  to 
raise  clouds  of  dust  high  into  the  atmosphere. 

599.  During  a  voyage   from   Richmond,'  Va.,  to   Rio 
Janeiro,  in  the  winter  of  1845-6,  Mr.  Thomas  Ewbank, 
of  the  U.  S.  Patent  Office,  met  with  many  instances  of 
the  falling  of  sea-dust,  and  traced  the  rich  and  peculiar 
hues,  that  at  times  adorned  the  clouds  and  sky,  to  the 
diffusion  of  this  fine  powder  throughout  the  intermediate 
atmosphere. 

600.  On  the  10th  of  January,  1846,  in  23°  33'  N.  Lat., 
and  34°  37'  W.  Long.,  he  observed  a  narrow  belt  of  slate- 
colored  sky  skirting  the  horizon,  while  upon  this  rested  a 
broad  band  of  vermilion,  interspersed  with  soft  dashes  of 
Indian  ink,  shaded  with  umber.     These  hues  changed, 
by  insensible  degrees,  into  a  bright  creatn-color,  and  this 
again  into  a  pale,  delicate  green,  which  deepened  in  tint 
as  it  approached  the  zenith,  while  over  all  floated  ainber- 
colored  clouds,  growing  richer  in  hue  and  smaller  in  size 
as  they  sunk  towards  the  horizon.        y 

What  is  the  extent  of  the  region  over  which  the  Cape  de  Verd  and 
Atlnntic  dust-storms  prevail  ? 

Whiit  is  the  opinion  of  Mr.  Darwin  as  to  the  origin  of  this  dust! 

Relate  in  full  the  account  given  by  Mr.  Ewbank  of  the  dust-storms 
that  he  observed  on  a  voyage  from  Richmond  to  Rio  Janeiro. 


BLOOD-RAINS.  245 

601.  Three  days  afterwards,  in  16°  07'  N.  Lat.,  and 
31°  13'  W.  Long.,  the  wind  blew  strongly  from  the  east, 
bearing  along  with  it  a  red,  impalpable  powder.     This 
minute  dust  was  seen  on  the  windward  side  of  the  sails, 
where  it  was  supposed  to  have  been  collecting  during  the 
two  previous  days.     It  was  extremely  fine,  and  could  only 
be  seen  by  bringing  the  loose  fibres  of  a  rope,  upon  which 
it  had  settled,  between  the  eye  and  the  sun,  when  its 
presence  and  color  were  readily  discerned. 

602.  The  sun  throughout  the  day,  as  well  as  the  moon 
at  night,  was  enveloped  in  a  haze,  which  was  supposed  to 
be  caused,  iij  some  measure,  by  the  dust  that  floated  in 
the  air.     The  captain  of  the  vessel,  who  had  noticed  this 
phenomena  before,  called  the  red  powder  African  sand. 

603.  During  the  two  following  days  the  heavens  pre- 
sented scenes  of  gorgeous  and  surpassing  beauty,  the  colors 
of  the  sky  and  clouds  ranging  through  emerald  green, 
pink,  purple,  crimson,  yellow,  chocolate,  umber,  and 
slate  ;  while  beneath  this  rich  and  varied  combination  a 
groundwork  of  the  purest  cream-color  extended,  giving 
tone  to  the  whole,  and  changing  in  tint  from  a  fawn-color 
to  a  pale  white. 

604.  On  the  16th  of  January,  in  7°  44'  N.  Lat.,  and 
28°  31'  W.  Long.,  the  red  dust  was  observed  to  accu- 
mulate  upon   the  vessel — an   old  sail,  looking   as  if  it 
had  been  painted  of  a  light  brick  color.     The  ship  at  this 
time  was  opposite  Soudan  and  Senegambia,  which  border 
on  the  great  African  desert,  whence  the  captain  supposed 
the  shower  to  come.     A  portion  of  the  dust  was  collected 
by  rubbing  a  piece  of  foolscap  paper  over  the  colored  sail. 

605.  A.  fall  of  dust,  accompanied  by  snow,  occurred  in 
the    month  of  February,  1850,  at  Olsterholz,  near  Det- 
mold,  in  Westphalia.     The  wind,  during  this  phenomena, 
blew  from  the  south-west.     The  dust  fell  so  thickly  as  to 
cover  the  earth  to  the  depth  of  one  eighteenth  of  an  inch. 

BLOOD-RAINS. 

606.  INSTANCES.     On  the  12th  of  August,  in  the  year 

What  is  said  of  the  dust-shower  that  occurred  at  Olsterholz  1 


246  MISCELLANEOUS    PHENOMENA. 

1222,  a  red  rain  fell  at  Rome  for  the  space  of  a  day  and 
a  night;  and  a  similar  event  occurred  at  Cremona  on  the 
3d  of  July,  1529.  In  1608  a  red  rain  descended  for 
several  miles  around  Aix,  in  France  ;  and  in  1623  an- 
other blood-rain  happened  at  Strasburg,  between  4  and  5 
o'clock  in  the  afternoon.  On  the  5th  and '6th  of  May, 
1711,  red  rain  fell  at  Orsio,  in  Sweden ;  and  a  shower 
of  this  nature  also  occurred  near  Genoa  in  the  year  1744. 

607.  A  very  remarkable  rain  of  this  character  fell  at 
Locarno,  in  Switzerland,  on  the  14th  of  October,  1755.  A 
warm  Sirocco  wind  was  here  blowing  at  8  o'clock  on  the 
morning  of  this  day,  and  two  hours  afterwards  the  air 
•was  filled  with  a  red  mist. 

At  4  o'clock  in  the  afternoon  a  blood-rain  descended, 
which  left  on  the  ground  a  reddish  deposit.  Nine  inches 
of  this  colored  rain  fell,  in  the  course  of  one  night,  over  a 
region  forty  square  German  leagues  in  extent.  It  even 
reached  Suabia,  on  the  northern  side  of  the  Alps ;  while 
amid  the  cold  heights  of  these  lofty  mountains  it  changed 
into  a  reddish  snow,  which  fell  to  the  depth  of  nine  feet. 

608..  The  red  matter  that  was  deposited  during  this 
shower  was  found,  by  actual  measurement,  to  be  in  some 
places  an  inch  deep,  or  one-ninth  part  of  the  quantity  of 
rain.  Upon  the  supposition  that  it  fell,  on  an  average,  to 
the  depth  of  only  one-sixth  of  an  inch,  twenty-seven 
hundred  cubic  feet  of  this  red  substance  must  have  cov- 
ered every  English  square  mile. 

609.  On  the  13th  of  November,  1755,  a  rc.d  rain  fell 
in  Russia,  Sweden,  Ultn,  and  on  the  Lake  of  Constance  ; 
and  on  the  9th  of  October,  1763,  a  similar  shower  de- 
scended at  Cleves,  Utrecht,  and  many  other  places  in 
Europe. 

610.  During  the  remarkable  phenomenon  that  occurred 
at  Gerace,  on  the  14th  of  March,  1813,  the  red  rain  pre- 
vailed over  a  great  extent  of  country,  falling  throughout 
the  two  Calabrias,  and  on  the  opposite  side  of  the  province 
of  Abruzzo,  in  the  kingdom  of  Naples. 


Relate,  in  detail,  the  several  instances  given  of  the  fall  of  blood- 
rain. 


BLACK    RAIN.  247 

611.  A  red  rain  likewise  fell  at  Sienna,  and  upon  the 
adjacent  country,  on  the  loth  of  May,  1830,  at  7  o'clock 
in  the  evening,  and  also  at  midnight.     The  weather  for 
two  days  previously  had  been  calm,  but  the  sky  was  over- 
cast with  dense,  reddish  clouds. 

612.  BLACK  RAIN.     The  material  that  mingles  with 
these  extraordinary  rains  is  not  always  of  a  red  hue,  but 
is  sometimes  of  a  dark  color,  and  imparts  an  inky  Slack- 
ness to  the   shower.     A  rain  of  this  kind  occurred  at 
Montreal,  in  Lower  Canada,  on  two  several  days  during 
the  month  of  November,  1819,  under  the  following  cir- 
cumstances : 

On  the  morning  of  the  21st  of  this  month  a  dense  gloom 
enveloped  the  city,  while  the  whole  atmosphere  was 
obscured  by  a  thick  haze,  of  a  dusky  orange  color,  and 
at  this  time  rain  descended  of  a  dark  inky  hue.  The 
weather  soon  after  became  pleasant,  and  continued  so 
until  the  following  Tuesday,  when  at  noon  the  whole  city 
was  again  shrouded  in  a  heavy,  damp  vapor,  so  dense 
that  it  became  necessary  to  light  candles  in  all  the 
houses. 

At  about  3  o'clock  in  the  afternoon  a  slight  shock  of  an 
earthquake  was  felt,  attended  by  a  noise  like  the  discharge 
of  distant  artillery. 

Soon  after,  when  the  darkness  was  the  deepest,  the 
gloom  was  dispelled  by  a  vivid  flash  of  lightning,  which 
was  followed  at  once  by  a  crashing  peal  of  thunder  ;  and 
this  was  succeeded  by  a  heavy  shower  of  thick,  black 
rain. 

613.  On  the  22d  of  April,  1846,  a  copious  Hack  rain 
fell  also  in  England,  in  the  towns  of  Dudley,  Stourport- 
Abberly,  and  Bewdley,  which  are  situated  in  the  northern 
part  of  Worcestershire. 

This  shower  lasted  from  11  o'clock  in  the  morning  till 
1  o'clock  in  the  afternoon,  the  rain  descending  so  abund- 
antly as  to  Uacken  the  waters  of  the  places  where  it  fell, 
and  darken  the  river  Severn. 


Give  an  nccount  of  the  Hack  rain  of  Montreal. 
Of  that  which  happened  in  Worcestershire. 


248  MISCELLANEOUS    PHENOMENA. 

614.  RED  HAIL.     A  storm  of  Ted  hail  is  stated  by 
Baron   Humboldt  to  have  once  occurred   at  Paramo,  in 
South  America,  between  Bogota   and   Popayan.     There 
l.kewise  fell  over  all  Tuscany,  on  the  14th  of  March,  1813, 
a  shower  of  hail  of  an  orange  hue. 

615.  BLACK  HAIL.     A  hail-storm  happened  in  Ireland 
on  the  14th  of  April,  1849,  which  deposited  upon   the 
ground  a  black,  inky  substance.     Some  of  this  dark  mat- 
ter was  collected  and  examined,  and  found  to  be  of  the 
same  nature  as  the  coloring  material  of  red  rains. 

STOBMS  OF  COLOEED  SNOW. 

616.  RED  SNOW.     One  of  the  most  remarkable  falls 
of  red  snow  on  record  is  that  which  has  already  been 
mentioned  (Art.  607),  as  occurring  simultaneously  with 
the  blood-rain  of  Lo'carno,  in  Switzerland,  when  snow  of  a 
/<vA//.s-A  hue  covered  the  neighboring  Alps  to  the  depth  of 
nine  feet. 

617.  On  the  5th  and  6th  of  March,  1808,  red  snow 
fell  for  the  space  of  three  nights  in  Carniola,  a  province 
of  Germany,  and  throughout  Carnia,  Cadore,  Belluno,  and 
Feltri,  to  the  depth  of  jive  feet  and  ten  inches. 

The  earth  had  been  previously  covered  with  white  snow, 
and  the  storm  of  colored  snow  was  succeeded  by  another, 
the  flakes  of  which  were  as  usual,  of  a  pure  and  brilliant 
white.  The  two  kinds  were  perfectly  distinct.  When  a 
portion  of  the  red  snow  was  melted  in  a  vessel,  and  the 
water  evaporated,  a  fine  rose-colored,  earthy  sediment 
remained  at  the  bottom.  Red  snow,  likewise,  fell  at  this 
time  on  the  mountains  of  the  Valtelline,  in  Switzerland,  at 
Brescia,  and  on  the  Tyrol. 

618.  During  the  dust- shower  and  blood-rain,  at  Ge- 
nicc,  red  snow  descended  over  a  wide  extent  of  country, 
embracing  the  two  Calabrias,  Tolmezzo,  and  the  Carnian 
Alps.     In  Tuscany  it  fell  of  an  orange  hue,  while  at 
Bologna  its  tint  was  a  brownish  yellow. 

What  instance  is  given  of  the  occurrence  of  red  hail] 
What  of  Waeihaill 

Where,  when,  and  under  what  circumstances  have  storms  of  colored 
snow  occurred  1 


NATURE    OF   THE    DUST.  249 

619.  On  the  15th  of  April,  1816,  colored  snow  fell  in 
Italy,  upon  Tonal,  and  on  other  mountains.     It  was  of  a 
brick-red  hue,  and,  when  melted  and  evaporated,  a  light 
and  impalpable  earthy  powder  remained. 

620.  A  storm  of  reddish  snow  took  place  on  the  31st 
of  March,  1847,  in  Puster  Valley,  in  the  Tyrol.     It  de- 
rived its  tint,  which  was  a  brownish  red,  from  a  fine 
colored  dust,  resembling  that  of  the  Atlantic  showers. 

621.  BLACK  SNOW.     A  few  years  ago  a  fall  of  black 
snow  occurred  in  New  Hampshire,  at  Walpole,  and  the 
adjoining  towns.     A  person  writing  to  the  Boston  Journal 
from  Walpole,  remarks,  in   relating  this   extraordinary 
phenomenon  :  "  I  send  you  some  writing,  written  with  the 
snow  as  it  fell,  and  with  a  clean  pen."     This  writing, 
according  to  the  editor  of  the  Journal,  was  perfectly  leg- 
ible, and  appeared  as  if  having  been  written  with  pale 
black  ink. 

622.  These  colored  snows  must  not  be  confounded  with 
those  already  described  in  Arts.  286,-  287,  and  288.     The 
snows  there  mentioned  are  white  before  their  fall,  and 
acquire  their  red  and  green  tints,  after  their  descent,  from 
the  presence  of  a  microscopic  plant  whose  cells  are  filled 
with   animalcules,    and   which,    even    in   Arctic   climes, 
spreads  itself  w.ith  extraordinary  vigor  over  fields  of  snow. 
On  the  contrary,  in  storms  of  colored  snow,  the  coloring 
matter  is  in  the  atmosphere,  and  the  snow  is  dyed  before 
itsfall. 

NATURE  OF  THE  DUST. 

623.  It  appears  from  the  microscopic  investigations  of 
eminent  observers,  and  especially  from  those  of  Ehrenberg, 
that  the  dust  which  causes  dust-storms,  and  produces  the 
phenomenon  of  blood-rains,  is  composed  both  of  organized 
and  unorganized  matter :  the  latter  being  portions  of 
various  minerals,  while  the  former  consists  principally 
of  the  shells  of  infusoria,  mingled  with  fragments  of  pet- 
rified plants  and  parts  of  insects. 

Are  these  colored  snows  the  same  in  character  as  those  already  cle« 
•cribed  in  Arts.  286,  287,  and  288  !     Why  not  1 

Of  what  is  the  dust  of  dust-storms  and  blood-rains  composed? 
Of  what  does  the  organic  matter  consist  1 
11* 


250          MISCELLANEOUS  PHENOMENA. 

It  may  not  be  amiss  to  explain  to  the  student  in  this 
place  the  meaning  of  the  term  infusoria. 

624.  INFUSORIA.     The  general  name  of  Infusoria  has 
been  given  to  those  minute  living  beings  which  can  only 
be  seen  by  the  aid  of  the  microscope.    On  account  of  their 
being  first  detected  in  vegetable  infusions,  they  are  termed 
infusoria;  and  since  they  are   exceedingly  small,  they 
have  also  received  the  appellation  of  animalcules,  or  little 
animals.      They  are  found  in  countless  myriads  in  all 
waters,  and  in  the  fluids  that  circulate  in  animal   and 
vegetable  bodies,  while  their  shells  and  eggs  are  dissem- 
inated by  the  winds  over  every  part  of  the  world. 

625.  More  than  eight  hundred  distinct  species  have 
been  discovered,  possessing  the  most  grotesque  and  sin- 
gular forms'.      Some   resemble   globes,   trumpets,   stars, 
boats,  and  coins;  others  assume  the  forms  of  eels  and 
serpents,  and  many  appear  in  the  shape  of  fruits,  neck- 
laces, pitchers,  wheels,  flasks,  cups,  funnels,  and  fans. 

Their  minuteness  is  almost  incredible,  for  the  monad, 
the  smallest  of  all  living  beings,  never  exceeds  in  length 
the  twelve  tlwusandth  part  of  an  inch.  A  single  shot, 
one-tenth  of  an  inch  ^n  diameter,  occupies  more  space 
than  seventeen  hundred  millions  of  these  atoms — each 
in  itself  a  perfect  being,  amply  endowed  with  vital  powers 
adapted  to  the  mode  and  range  of  its  existence. 

626.  STRUCTURE.     The  outer  covering  of  the  infuso- 
ria is  of  two  kinds  ;  t\\e  first  is  soft  and  yielding,  resem- 
bling the  skin  of  the  leech  and  slug  ;  but  the  second  is  a 
fine,  transparent  shell,  possessing  a  flexibility  like  horn. 
Those  animalcules  that  are  protected  by  the  latter  integ- 
ument are  termed  loricated,  from  the  Latin  word  lorica, 
a  shell ;  while  the  name  illoricated,  or  shelless,  is  assigned 
to  those  which  are  invested  with  the  softer  covering. 

The  material  that  composes  the  shells  varies  in  diflereni 
species.  In  many  kinds  it  consists  entirely  of  flint,  and 


Describe  the  infusoria — the  number  of  their  species — their  minute* 
ness. 

What  is  said  respecting  their  structure  7 


ITALIAN    AND    CALABRIAN    DUST-SHOWERS.          251 

in  others  of  lime,  united  with  oxide  of  iron.     In  some 
cases  it  is  combustible. 

627.  When  the  loricated  infusoria  die,  their  shells  re- 
main   undecayed   for    ages,    often    congregated   in    such 
countless  myriads  as  to  form  large  portions  of  the  earth's 
surface. 

The  city  of  Richmond,  in  Virginia,  is  built  upon  an 
extensive  bed  of  flinty  marl,  from  twelve  to  twenty  feet 
in  thickness,  filled  with  fossil,  infusorial  shells  /  and  it 
is  stated  by  geologists,  that  nearly  half  of  the  bulk  of  all 
the  chalk  of  Northern  Europe  is  composed  of  the  fossil 
remains  of  animalcules,  and  other  minute  shells.  They 
are  mingled  with  the  mud  that  forms  the  bed  of  the  Arctic 
Ocean ;  they  float  with  the  iceberg  in  all  its  wanderings, 
and  lie  loosely  scattered  over  the  surface  of  every  land. 
These  hieroglyphics  of  nature  are  interpreted  by  the  aid 
of  the  microscope.* 

628.  THE  ITALIAN  DUST-SHOWER  OF  1803,  AND  THE 
CALABRIAN  OF  1813.     In  the  dust  which  fell  in  Italy 
during  the  month  of  March,  1803,  forty-nine  species  of 
organic  structures  were  discovered,  and  sixty-four  in  that 
which  descended  at  Gerace,  in  Calabria,  in  1813.    Thirty- 
nine  species  in  the  Italian  dust-shower,  and  fifty-one  in 
the  Calabrian,  are  identical  with  those  discovered  in  more 
recent  dust-storms.     It  is  worthy  of  remark,  that  these 
two  storms,  though  ten  years  apart,  have  no  less  than 
twenty-eight  species  in  common,  and  in  ooth  nearly  all 
the    species   are    of  fresh-water   origin.      Among   the 
numerous  infusorial  shells,  four  South  American  forms 
were  discovered  ;  of  these,  one  occurs  in  Peru,  another  in 
Surinam,  and  the  remaining  two  belong  to  Chili.     No 
animalcular  structures  were  found  exclusively  African. 

629.  ATLANTIC  AND  CAPE  DE  VERD  DUST.     The  dust 


"When  the  loricated  infusoria  die,  what,  becomes  of  their  shells  1 
Of  what  did  the  dust  consist  which  fell  in  the  Italian  and  Calabrian 
dust-storms  1  

*  For  further  information  on  the  subject  of  Living  and  Fossil  Infusoria,  gee  "  Viewi 
of  the  Microscopic  World,"  by  the  author;  published  by  Farmer,  Brace  &  Co.,  New 
York. 


252  .MISCELLANEOUS    PHENOMENA. 

that  was  collected  -by  Mr.  Darwin  on  the  Atlantic,  in  N. 
Lat.  17°  43',  W.  Long.  26°,  and  at  the  distance  of  about 
Jive  hundred  miles  from  the  African  coast,  was  submitted 
to  the  examination  of  Ehrenberg,  who  discovered  that 
one-sixth  part  of  it  was  composed  of  the  flinty  shells  of 
fresh  water  and  land  infusoria,  and  of  silicious  fossil 
plants.  There  were  eighteen  species  of  the  former,  and 
as  many  of  the  latter.  Of  the  animalcular  remains,  the 
greater  part  were  European ;  one  species  was  decidedly 
of  South  American  origin,  and  another  probably ;  but 
there  were  none  that  belonged  exclusively  to  Africa.  In 
the  opinion  of  Ehrenberg,  the  two  South  American  spe- 
cies were  either  brought  from  that  country  by  the  upper 
winds  of  the  atmosphere,  or  from  some  other  locality  which 
is  yet  unknown. 

630.  In  the  dust  of  several  other  showers,  which  occur- 
red between  the  years  1834  and  1838,  some  at  St.  Jago, 
and  some  on  the  neighboring  ocean,  numerous  organized 
structures  were  discovered,  thirty  of  which  were  different 
from  those  detected  in  the  dust  just  described.     Among 
these  were  the  shells  of  a  few  South  American  infusoria, 
and  one  beautiful  microscopic  shell,  termed  the  Polytha- 
lamia*  or  many -chambered  shell.     A  single  species  was 
observed  that  occurs  in  the  Isle  of  France  ;  but  none  of 
the  forms  were  recognized  as  peculiarly  African. 

631.  Some  of  the  dust  collected  by  Mr.  Ewbank,  on 
bis  voyage  to   Rio  Janeiro,  was   examined  by  Professor 
Bailey,  of  West  Point ;  but  he  was  unable  to  discover  in 
it  any  thing  besides  irregular,  inorganic,  mineral  frag- 
ments.    He  believes,  however,  that  more  interesting  results 
would  have  been  obtained  if  the  dust  had  been  gathered 
with  greater  care.     The  entire  number  of  distinct  organic 

Relate,  in  full,  what  is  said  respecting  the  composition  of  the  Atlan- 
tic and  Cape  de  Verd  dust. 

What  is  said  respecting  the  dust  of  several  other  showers  1 

Were  any  of  the  forms  distinctively  African T 

TV.-re  any  organisms  discovered  in  the  dust  collected  by  Mr.  Ew- 
bank  ? 

What  is,  however,  the  opinion  of  Prof.  Bailey  1 

*  *i>om  polus,  (Greek,)  many,  and  thalamits,  (Latin,)  a  chamber. 


SIROCCO    DUST. 


253 


forms  hitherto  discovered  in  the  Cape  de  Verd  and  Atlan- 
tic dust-storms  is  sixty-seven. 

*  632.  SIROCCO  DUST.  The  dust  that  fell  at  Malta  on 
the  15th  of  May,  1830,  afforded  forty-three  distinct  organ- 
ized forms  ;  of  these  there  were  fifteen  infusorial  struc- 
tures, twenty-one  kinds  of  minute,  petrified  plants,  and 
seven  of  Polythalamia.  The  species  of  animalcules 
were,  for  the  most  part,  identical  with  those  discovered  in 
the  Cape  de  Verd  and  Atlantic  dust-showers. 


MICROSCOPIC   ORGANISMS  OP  THE  1.TOK8  DUST-SHOWKR. 


(Fossil  Infusoria.) 

One  form  was  noticed  belonging  peculiarly  to  Chili,  but 
none  were  found  distinctively  African. 

State  the  entire  number  of  organic  forms  hitherto  detected  in  th« 
Cape  de  Verd  and  Atlantic  dust-stormi 


254 


MISCELLANEOUS    PHENOMENA. 


633.  The  Sirocco  dust  that  fell  at  Lyons,  on  the  17th 
of  October,  1846,  was  so  rich  in  organic  remains  that  they 
constituted  one-eighth  part  of  its  mass.  They  consisted 
of  numerous  species  of  infusoria  and  of  petrified  plants, 
mingled  with  a  few  kinds  of  Polythalamia,  and  minute, 
vegetable  fragments.  The  species  were  nearly  all  of 
fresh-water  origin,  one-seventh  only  being  marine.  In 
figures  45  and  46  are  delineated  the  various  microscopic 

Fig.  46. 


MICROSCOPIC  OEG 


(Fossil  Plants.) 


organisms  which  were  discovered  in  this  dust.  The  most 
remarkable  circumstance  respecting  it  is  the  fact,  that, 
notwithstanding  its  general  resemblance  to  the  dust  of 
the  Atlantic  showers,  which  has  always  exhibited  nothing 
but  dead  and  empty  infusorial  shells,  this,  on  the  con- 
trary, was  found,  in  many  cases,  to  contain  a  species  of 

Describe  the  nature  of  the  Sirocco  dust  that  fell  at  Malta  and  at 
Lyons. 

What  is  remarkable  respecting  the  Lyons  shower  1 


ORGANISMS    OF    DUST-SHOWERS.  255 

infusoria  which  was  distinctly  seen  to  be  filled  with  green 
ovaries,  or  egg-sacks,  and  consequently  was  capable  of 
Ufe. 

634.  The  dust  collected,  in  the  preceding  instances, 
from  the  Cape  de  Verd,  Atlantic,  and  Sirocco  showers, 
being  nine  in  all,  afforded  119  distinct  organisms.     Of 
these   there  were   fifty-seven    species  of  infusoria,  and 
eight  of  Polythalamia  ;  forty-six  kinds  of  fossil  plants, 
together  with  particles  of  seven  kinds  of  plants,  and  one 
fragment  of  an  insect.     Only  seventeen  of  these  organ- 
isms were  marine  j  while  102,  six-sevenths  of  the  whole, 
consisted  of  fresh-water  species.     In   all  these  showers 
the  dust  exhibited  no  indications  whatever  of  volcanic 
origin. 

635.  In  three  dust-showers  which  occurred  in  the  years 
1847  and  1848 — t\\e  first  in  Salzburg,  the  second  in  Ara- 
bia, and  the  third  in  Silesia  and  Lower  Austria — similar 
fresh-water  organisms  were  detected.     The  same  South 
American  species  were  here  found,  as  in  other  showers, 
without  any  characteristic  African  forms. 

636.  The  red  snow  that  fell  in  the  Tyrol  on  the  31st 
of  March,  1847,  afforded  sixty-six  different  organic  forms. 
Of  these,  twenty-two  were  infusorial  structures,  twenty- 
eight  jfom£  plants,  two  polythalamia,  and  thirteen  par- 
ticles of  plants.     There  was  also  one  fragment  of  an 
insect.     The  greater  part,  by  far,  of  all  these  species, 
were  of  land  origin,  two  only  being  marine. 

A  remarkable  resemblance  exists  between  the  coloring 
matter  of  this  shower  and  the  dust  of  the  Atlantic,  Geno- 
ese, and  Lyons  storms,  not  only  in  its  hue,  but  in  its 
composition  /  for  out  of  these  sixty-six  structures,  forty- 

How  many  distinct  organisms  were  discovered  in  the  dust  of  nine 
showers  1  Describe  the  several  kinds. 

Was  there  any  trace  of  volcanic  duet  in  these  showers  1 

What  is  observed  respecting  the  dust-storms  which  happened  in 
the  years  1847  and  1848  1 

What  organisms  were  detected  in  the  red  snow  of  the  Tyrolese 
etorm  1 

What  resemblance  was  observed  between  the  coloring  matter  of  this 
Bhower  and  the  dust  that  fell  on  the  Atlantic,  at  Genoa,  and  at 
Lyons  1 


256          MISCELLANEOUS  PHENOMENA. 

six  are  found  in  the  Atlantic  and  Sirocco  dust ;  and  twelve 
species  of  infusoria  and  twenty  of  fossil  plants  are  com- 
mon to  all. 

637.  In   the   dust   that   fell,   mingled    with   snow,    at 
Olsterholz,   in    the  year  1850,  Ehrenberg  detected  fifty 
organic  forms,  forty  of  which  he  had  previously  observed 
in  the  dust  of  other  showers.     The  remaining  ten  species 
had  never  been  before  discovered  in  atmospheric  dust. 

638.  NUMBER  or  DISTINCT  ORGANISMS  DISCOVERED. 
In  the  dust  of  the  various  showers  examined  by  this  dis- 
tinguished naturalist,  no   less   than  320  distinct  species 
of  organisms  were  discovered.     Of  these,  five  only  were 
of  marine  origin,  and  fourteen  were  forms  peculiar  to 
America. 

639.  NUMBER   AND    EXTENT  OF   DUST-STORMS    AND 
BLOOD-RAINS.     According  to   the  researches  of  Ehren- 
berg, 340  instances  of  dust-storms  and  blood-rains  are 
mentioned  in  history  and   in  the    annals  of  science,  of 
which  81  took  place  before  the  Christian  era,  and  259 
after  it.     These  remarkable  phenomena  extend  through- 
out the  world,  occurring  on  the  ocean,  on  all  the  conti- 
nents, and  even  in  Australia.     They  appear,  however,  to 
prevail  most  within  a  zone,  extending  from  that  part  of 
the  Atlantic   off  the  west    coast   of   Middle  and   North 
Africa,  along  in  the  direction  of  the  Mediterranean  Sea, 
reaching  a  short  distance  north  of  this  sea,  and  continuing 
into  Asia  between  the  Caspian  Sea  and  the  Persian  Gulf, 
perhaps  to  Turkistan,  Kaschgar,  and  even  China:  they 
seldom  happen  as  far  north  as  Sweden  and  Russia. 

This  zone,  according  to  the  observations   of   Captain 
Tuckey,  has  a  breadth  of  1800  miles. 


"What  organic  forms  were  discovered  in  the  dust  that  fell  at  Olster- 
holz 1 

How  many  distinct  organisms  have  been  detected  by  Ehrenberg  iu 
the  dust  of  numerous  dust-stories  1  What  is  said  respecting  their 
origin  1 

What  is  the  number  and  extent  of  dust-storms  and  blood-rains,  accord- 
to Ehrenberg  7 

avail  most  ? 


ig  to  iiihrenoerg ! 
Where  do  they  appear  to  prevail  r 
What  is  the  breadth  of  this  zone  1 


ORIGIN    OF    THE    DUST.  25V 

640.  THEIR  PERIODICITY.  These  phenomena  occur 
most  frequently  during  the  first  half  of  the  year  ;  for 
out  of  199  showers,  whose  dates  are  ascertained,  118 
happened  between  January  and  July,  and  81  between 
July  and  December.  The  distribution  of  the  showers 
through  the  several  months  is  as  follows : 

January,  27.  July,  9. 

February,  14.  August,  17. 

March,  23.  September,  7. 

April,  18.  October,  18. 

May,  18.  November,  16. 

June,  18.  December,  14. 

641.  ORIGIN  OF  THE  DUST.  The  color  and  nature 
of  this  dust ;  the  circumstance  that  a  great  quantity  of 
earthy  matter  sometimes  falls  in  a  single  shower,  as  in 
that  of  L^ons  ;  and  the  fact  that  dust-storms  arid  blood- 
rains  have  occasionally  happened  from  the  time  of  Homer 
(900  B.C.)  to  the  present  day,  have  led  Ehrenberg  to 
advance  a  most  extraordinary  hypothesis.  He  believes 
that  these  phenomena  are  not  to  be  traced  to  mineral  mat- 
ter belonging  to  the  earth's  surface ;  neither  to  masses  of 
dust  revolving  in  space,  like  the  meteoric  matter  of  Chaldni 
(Art.  555) ;  nor  yet  to  the  influence  of  atmospheric  cur 
rents,  such  as  the  trade-winds  and  harmattan,  carrying 
the  dust  of  the  earth  aloft  into  the  air ;  but  to  some  gen- 
eral law,  as  yet  unknown,  according  to  which  infusoria, 
and  other  living  organisms,  exist  and  are  propagated  in 
the  upper  regions  of  the  atmosphere. 

The  locality  which  constitutes  the  dwelling-place  of  these 
organisms  he  imagines  to  be  of  vast  extent,  and  to  be  sit- 
uated at  the  height  of  about  14,000  feet  above  the  sea- 
level. 


In  what  parts  of  the  year  do  these  phenomena  most  frequently 
occur  1 

How  are  they  distributed  through  the  months  1 

What  are  the  views  of  Ehrenberg  respecting  the  origin  of  the  dost 
that  fa  1ft  in  these  singular  storms  1 

Where  does  he  suppose  the  abode  of  these  organisms  to  be  sit- 
uated 1 


258          MISCELLANEOUS  PHENOMENA. 

642.  The  apparent  periodicity  of  the  showers  he  ac- 
counts for  by  supposing  that  this  cloud  of  organisms  lies  in 
the  region  of  the  trade-winds,  and  suffers  partial  and  pe- 
riodical deviations. 

643.  In  the  present  imperfect  state  of  our  knowledge  in 
regard  to  these  phenomena,  it  would  be  highly  unsafe  to 
adopt  this  singular  hypothesis. 

Both  the  organic  and  inorganic  matter  contained  in 
these  storms  are  terrestrial  in  their  nature,  and  the  at- 
mospheric currents  are  most  probably  the  agents,  which 
elevate  this  dust  from  the  surface  of  the  globe,  and  bear  it 
along  to  distant  regions. 

644.  The  opinion,  that  the  Atlantic,  Cape  de  Verd,  and 
Sirocco  dust  comes  from  the  deserts  of  Africa,  is  incon- 
sistent with  certain  known  facts  respecting  it,  and  has 
therefore  not  been  universally  adopted.     For  instance,  the 
color  of  this  dust  is  red,  while  the  sand  of  the  African 
Saharas  is  white  and  gray  /  and  we  have  also  seen  that 
none  of  the  organized  forms  which  it  contains  are  peculiar 
to  Africa ;  while  many  of  them  are  distinctively  South 
American. 

645.  It  is  the  belief  of  Lieutenant  Maury,  that  the  red 
powder,  which  falls  in  these  dust-storms,  is  brought  by  an 
upper  wind  from  South  America  to  Africa ;  where  it  de- 
scends and  becomes  the  lower  trade-wind,  which  dissem- 
inates the  dust  throughout  the  regions  where  it  blows.     It 
is  not  improbable  that  a  portion  of  this  dust,  carried  on- 
wards by  the  higher  current,  falls  within  the  sweep  of  the 
Sirocco — a  circumstance  which  will  fully  explain  the  sim- 
ilarity that  exists  between  the  Sirocco  and  Atlantic  dust. 


*    How  does  he  account  for  the  apparent  periodicity  of  these  showers 

and  storms  * 

Are  there,  at  present,  sufficient  reasons  for  adopting  this  hypothesis  1 
"What  is  the  nature,  both  of  the  orgHnic  and  inorganic" bodies,  which 

constitute  this  dust  1 

"How  are  they  probably  raised  into  the  atmosphere  1 
What  reasons  exist  for  believing  that  the  Atlantic,  Cape  de  Verd, 

and  Sirocco  dust  does  not  come  from  Africa  1 

What  is  the  opinion  of  Lieutenant  Maury  upon  this  point  1    » 
How  can  the  similarity  in  the  nature  of  the  Sirocco  and  Atlantic 

dust  be  explained  ! 


VOLCANIC    SHOWERS.  259 

VOLCANIC  SHOWERS. 

646.  The  fall  of  ashes  and  dust  soon  after  the  eruption 
of  volcanoes,  is    a  phenomenon  entirely  different  from 
dust-storms  and  Hood-rains  /  for  the  materials  which  are 
precipitated  in  volcanic  showers  contain  no  organic  forms* 
and  are  easily  traced  to  their  source. 

647.  CAUSE.     The  mighty  energies  that  are  at  work, 
when  a  volcano  is  in  full  action,  carry  up  the  lighter  por- 
tion of  the  ejected  matter  high  into  the  air;  it  is  then 
borne  along  by  the  upper  winds,  and  at  length  falls,  in 
showers,  in  regions  often  far  remote  from  the  burning 
crater. 

648.  INSTANCES — JORULLO.     During  the  eruption  of 
Jorullo,  in  Mexico,  which  began  on  the  28th  of  Septem- 
ber, 1759,  the  sky  was  darkened  with  clouds  of  dust  that 
afterwards    fell    at   Queretaro,   100  miles   distant  ;    and 
during  another  eruption  of  the  same  volcano  in  1819,  dust, 
to  the  depth  of  six  inches,  descended  in  the  streets  of, 
Guanaxuato,  at  the  distance  of  160  miles. 

649.  SOUFFRIERE.     One  of  the  most  remarkable  vol- 
canic dust-showers  on  record  is  that  connected  with  the 
eruption  of  the  Souffnere  mountain,  in  the  island  of  St. 
Vincent,  which  occurred  on  the  30th  of  April,  1812. 

650.  On  the  27th  of  this  month  the  volcano,  which  had 
been  slumbering  for  a  hundred  years,  again  burst  forth, 
showering  down  sand,  mixed  with  ashes  and  gritty,  cal- 
cined particles   of  earth.    -This  dust,  driven  before  the 
wind,  darkened  the  air  like  a  cataract  of  rain,  and  covered 
the  ridges,  woods,  and  cane-lands  with  light  grey-colored 
ashes,  resembling  snow.     As  the  activity  of  the  volcano 
increased,  this    continual   shower   extended  farther  and, 
farther,  destroying  every  trace  of  vegetation. 

651.  For  three  days   the  appearance  of  the  burning 
mountain   grew   more    awful    and    portentous,   when    at 
length,  on  the  night  of  the  30th,  a  most  terrific  eruption 

State  -what  is  said  respecting  volcanic  showers — their  cause. 
Give  an  account  of  the  showers  attending  the  eruptions  of  Jorullo. 
What  remarkable  shower  of  this   kind  is  next  mentioned]     De« 
ecribe  it 


260          MISCELLANEOUS  PHENOMENA. 

took  place.  From  the  midst  of  a  lofty  pyramid  of  flame 
issued  streams  of  glowing  lava,  which,  pouring  down  the 
sides  of  the  mountain,  flowed  in  torrents  to  the  sea ;  while 
the  sullen  roar  of  these  burning  rivers  was  swelled  by  the 
thunderings  and  loud  explosions  of  the  crater.  Stones, 
fire,  ashes,  and  calcined  masses  rained  down  for  hours, 
and  earthquake  following  earthquake,  almost  incessantly, 
the  whole  island  undulated  like  water  shaken  in  a  bowl. 

652.  On  the  next  day,  the  air  was  so  filled  with  vol- 
canic dust,  that  it  was  dark  at  8  o'clock  in  the  morn- 
ing ;  a  dense  haze  shrouding  sea  and  land.     Most  of  the 
plantations  in  the  vicinity  of  the  Souffriere  mountain  were 
covered  ten  or  twelve  inches  deep  with  dust  and  stones. 

653.  But  the  effects  of  this  eruption  were  not  confined 
to  this  island.     During  the  night  of  the  30th  the  terrific 
explosions  of  the  volcano  were  heard  as  fat  as  Barbadoes, 
which  is  situated  seventy  'miles  due  east  from  St.  Vin- 
cent.    On  the  next  morning,  at  4  o'clock,  the  atmosphere 
at  Barbadoes  was  bright  and  clear,  but  at  6  o'clock  the 
sky  was  obscured  by  thick  clouds,  from  which  issued  in 
torrents,  like  rain,  particles  of  volcanic  matter  fine.r  than 
sand.     At  8  o'clock,  an  appalling  darkness,  as  intense  as 
that  which  prevails  in  the  depth  of  a  stormy  night,  over- 
spread the  island,  and  continued  till  noon,  but  the  showers 
of  dust  still  descended  at  intervals  until  7  o'clock  in  the 
evening. 

654.  This  dust  descended  to  the  depth  of  two  inches, 
and,  according  to  the  computation  of  observers,  an  average 
•weight  of  40,000  Ibs.  rested  upon  every  acre  on  which  it 
fell.     Vessels  at  sea,  some  300  miles,  and  others  500  to 
the  windward  of  St.  Vincent,  had  their  decks  covered  with 
this  volcanic  dust.     (Art.  103.) 

655.  TOMBORO.     Still  more  surprising  was  the  dust- 
shower,  caused  by  an  eruption  of  Tomboro,  a  volcano  sit- 
To  -what  other  island  did  this  dust  extend  1 

How  far  is  it  from  St.  Vincent  ? 

How  was  the  atmosphere  of  Barbadoes  affected  by  this  volcanic 
dust? 

At  what  distance  from  St.  Vincent  were  vessels  covered  with  thi* 
dust? 


ERUPTION    OF    COSIGUINA.  261 

uated  in  the  island  of  Sumbawa,  which  lies  east  of  Java, 
and  south  of  Borneo. 

656.  The  eruption  occurred  on  the  12th  of  April,  1815. 
According  to  Sir  Stamford  Raffles,  who  was  then  governor 
of  Java,  the  roar  of  the  volcano  was  distinctly  heard,  in 
one  direction,  at  Ternate,  720  miles  distant  from  Tomboro, 
and  in  another,  as  far  as  Sumatra,  at  the  distance  of  970 
miles. 

657.  Such  vast  clouds  of  ashes  and  dust  were  ejected, 
that  the  day  at  Sumbawa  was  as  dark  as  the  blackest 
night ;  these,  rising  within  the  sweep  of  the  higher  winds, 
were  carried  in  immense  quantities  to  Java,  300  miles 
distant,  and  hung  like  a  pall  over  the  island.     At  Macas- 
sar, 250  miles  from  Sumbawa,  a  total  darkness  prevailed 
long  after  the  sun  had  risen,  and  volcanic  dust  fell  an  inch 
and  a  half  deep.     Some  of  the  ashes  were  carried  even  as 
far  as  the  island  of  Amboyna,  which  is  situated  800  milea 
from  Tomboro. 

658.  Near  Sumbawa,  such  quantities  of  lava,  cinders, 
and  ashes  fell  into  the  sea,  that  they  formed  a  cake  on  the 
surface  two  feet  in  thickness,  and,  for  miles  around  the 
island,  the  ocean    was    so   completely  covered  with   this 
floating  matter,  that  the  progress  of  ships  was  materially 
impeded. 

659.  COSIGUINA.     During  the  eruption  of  the  volcano 
of  Cosiguina,  in  Nicaragua,  on  the  20th  of  January,  1835, 
immense  clouds  of  dust  darkened  the  sky,  and  were  borne 
by  the  winds  to  a  great  distance. 

660.  At  Union,  a  sea-port  on  the  western  shore  of  the 
bay  of  Conchagua,  and  the  nearest  place  to  the  volcano 
of  any  importance,  showers  of  dust  fell  at  intervals  from 
the  20th  to  the  27th  of  January.     It  descended  in  the 
form  of  a  fine  powder  like  flour,  and  in  such  quantities  as 


Describe  the  eruption  of  Tomboro. 

What  is  said  respecting  the  ejected  ashes  and  dustl 

How  did  they  affect  the  atmosphere,  and  how  far  were  they  car- 

Ified  ] 

What  is  eaid  of  the  condition  of  the  sea  around  Sumbawa  1 

Give  an  account  of  the  showers  of  ashes  and  dust  caused  by  th« 

eruption  of  the  volcano  of  Cosiguina. 


262          MISCELLANEOUS  PHENOMENA. 

to  cover  the  earth  to  the  depth  of  Jive  inches;  causing, 
for  the  space  of  forty-three  hours,  so  intense  a  darkness 
that  lights  and  torches  were  needed,  and  even  these  were 
insufficient  to  render  objects  clearly  visible. 

661.  At  Leon,  the  capital  of  Nicaragua,  showers  of 
ashes  and  dust  descended  on  the  23d  of  January  to  the 
depth  of  nine  inches ;  and  at  Nacaome  the  falling  dust 
was  mingled  with  coarse  sand,  which,  together,  formed  a 
layer  upon  the  surface  of  the  ground  seven  or  eight  inches 
deep.     Some  of  the  ashes  ejected  during  this  eruption 
were   carried  even  as   far  as  Kingston,  Jamaica,  seven 
hundred  and  thirty  miles  distant  from  Cosiguina.    (Art. 
103.) 

662.  YELLOW   RAINS — POLLEN-RAINS.      Showers   of 
rain,  mingled  with  recent  vegetable  matter,  consisting  of 
the  pollen  of  various  plants,  have  been  noticed  for  a  con- 
siderable period  of  time.     A  shower  of  this  kind  once  fell 
at  Lund,  in  the  south  ^  Sweden,  the  pollen  having  been 
borne  by  the  wind,  from  a  forest  of  fir,  thirty-five  miles 
distant.     A-  similar  rain  fell  on  the  lake  of  Zurich,  in  the 
year  1677,  and  another  of  the  same  nature,  at  Bordeaux, 
in  1761.     During  a  thunder-storm  at  Banff,  in  Scotland, 
on  the  9th  of  June,  1835,  a  shower  of  yellow  rain  de- 
scended, which  tinged  the  waters  of  the  river  Devern  and 
of  the  neighboring  pools  with  the  same  color.     The  hue  in 
this  case,  as  in  the  preceding  instances,  was  derived  from 
the  pollen  that  was  mingled  with  the  rain. 

663.  A  few  years  ago,  a  rain  of  this  color  extended  over 
the  western   and   south-western   regions   of  the   United 

'States.  '  At  Carrollton,  Ohio,  the  ground,  after  the  rain, 
was  covered  with  a  yellow  substance;  and  the  same  phe- 
nomenon was  likewise  noticed  at  this  time  at  Zanesville, 
Cincinnati,  Louisville,  St.  Louis,  Natchez,  and  New  Or- 
leans. The  hue  of  this  rain  is  undoubtedly  to  be  attrib- 
uted to  the  presence  of  pollen. 

664.  GOSSAMER-SHOWER.     A  phenomenon  of  a  very 
extraordinary  nature  was  observed  by  the  Rev.  Gilbert 


"What  is  a  pollen-rain  1     Give  the  several  instances. 


GOSSAMER-SHOWER.  203 

White,  on  the  21st  of  September,  1741,  and  of  which  an 
account  is  given  in  his  charming  "  Natural.  History  of 
Selborne."  It  appears  from  the  statement  of  this  gentle- 
man, that  on  the  day  just  mentioned,  at  about  9  o'clock  in 
the  morning,  a  shower  of  cobwebs  ',  falling  from  very  ele- 
vated regions,  was  observed,  and  which  continued  to 
descend,  without  any  interruption,  till  the  close  of  the 
day. 

665.  These  webs  were  not   single,  filmy  threads,  but 
perfect  flakes  or  rags  —  some  being  nearly  an  inch  broad 
and  five  or  six  long,  which  fell  with  a  degree  of  velocity 
that  showed  they  were  considerably  heavier  than  the  at- 
mosphere. 

On  every  side,  as  the  observer  turned  his  eyes,  he  might 
behold  a  continual  succession  of  fresh  flakes  falling  into 
view,  and  twinkling  like  stars  as  they  reflected  the  rays 
of  the  sun.  This  singular  shower  was  noticed  at  Bradley, 
Selborne,  and  Alresford,  three  places  which  lie  in  a  kind 
of  triangle,  the  shortest  of  whose  sides  is  about  eight 
miles  long.  Whether  it  extended  farther,  is  not  certainly 
known. 

666.  At  Selborne,  a  gentleman,  who  observed  this  phe- 
nomenon while  taking  his  morning  ride,  supposed,  at  first, 
the  falling  flakes  to  have  been  blown,  like  thistle-down, 
from  the  fields  above,  and  imagined  that  he  would  be  free 
from  the  shower  when  he  had  gained  the  summit  of  a  hill 
that  rose  near  his  house.     But  upon  reaching  this  point, 
300  feet  higher  than  his  residence,  he  found  the  webs,  in 
appearance,  still  as  much  above  as  before  —  still  descending 
into  sight  in  a  constant  succession,  and  twinkling  brightly 
in  the  sun. 

Neither  before  nor  after  was  any  such  fall  observed  in 
these  places,  but  on  this  'day.  the  gossamer  flakes  hung  so 
thickly  in  the  trees  and  hedges,  that  a  person  might  have 
gathered  them  by  ~basketfuls. 

667.  In  explanation  of  this  curious  phenomenon,  Mr. 


Give  an  account  of  the  gossamer-shower  described  by  Rev.  Gilbert 
White. 
How  does  he  explain  this  phenomenon  ? 


264        •  MISCELLANEOUS  PHENOMENA. 

White  observes,  that  the  gossamer  threads,  which  £ks»t  in 
the  air,  are  .the  production  of  small  spiders,  that  swarm  in 
the  fields  in  fine  weather  in  autumn,  and  have  the  power 
of  shooting  out  webs  so  as  to  render  themselves  buoyant 
and  lighter  than  the  air.  If  taken  in  the  hand,  they  will 
run  along  the  fingers,  throw  out  a  web,  and  sail  aloft.  He 
supposes,  that,  possibly,  these  spiders,  with  their  webs,  are 
carried  up  into  the  higher  regions  of  the  atmosphere  by 
the  warm  and  light  currents  of  air  which  ascend  from  the 
earth  ;  and  that  while  thus  elevated  they  have  the  power, 
perhaps,  of  thickening  their  webs — as  some  naturalists  sup- 
pose— thus  rendering  them  heavier  than  the  atmosphere, 
when  of  course  they  must  fall,  and  will  thereby  occasion, 
if  they  descend  simultaneously  in  large  flakes  and  in 
great  abundance,  a  gossamer-shower. 

668.  Dr.  Lister  ascended  one  day,  when  the  air  was 
very  full  of  gossamer,  to  the  highest  part  of  York  Minster, 
and  still  found  these  filmy  threads  floating  far  above  him. 


CHAPTER   II. 
DRY  FOG  AND  INDIAN-SUMMEB  HAZE. 

669.  DRY  FOG.     A  peculiar  haze  sometimes  pervades 
the  atmosphere,  which  has  received  from  meteorologists 
the  name  of  dry  fog.     It  is  different  from  humid  mist,  for 
it  not  unfrequently  prevails  when  no  visible  vapor  exists 
in  the  air,  and  during  seasons  of  great  heat. 

670.  When  this  phenomenon  occurs,  the  sky,  although 
it  may  be  perfectly  free  from  clouds,  has   lost  its   fine 
azure  tint,  and  is   dull  and  discolored.     Terrestrial  ob- 
jects at  a  distance,  and  of  a  deep  color,  are  lost  to  view, 
arid  appear  as  if  covered  with  a  blue  veil.     The  sun  loses 


What  did  Dr.  Lister  observe  1 
What  is  dry  f<>«  ! 

Ho\v  does" this  phenomena  affect  the  appearance  of  celestial  ana 
terrestrial  objects'! 


DRY    FOGS.  265 

its  brilliancy,  even  when  high  in  the  heavens,  and  its  light 
is  of  a  reddish  hue.  As  it  approaches  the  horizon  it  as- 
sumes a  blood-red  color,  and  may  be  gazed  at  without 
dazzling  the  eyes.  At  times,  the  haze  is  even  so  thick  that 
the  solar  orb  ceases  to  be  visible  before  it  has  descended 
below  the  horizon. 

671.  INSTANCES.  In  the  year  1782,  a  remarkable  fog  of 
this  kind  occurred,  extending  over  Europe  from  Lapland  to 
the  Mediterranean.  It  was  succeeded  the  next  year  by 
another  still  more  extraordinary.  This  fog,  known  as  the 
dry  fog  of  1783,  produced  a  great  sensation  throughout 
Europe.  According  to  Kaemtz,  its  intensity  was  such, 
that  in  some  places  objects  at  the  distance  of^t/iree  miles 
could  not  be  distinguished.  Sometimes  they  appeared 
blue,  or  else  surrounded  with  vapor.  The  sun,  shorn  of 
its  beams,  appeared  of  a  fiery  red,  and  at  noon  could 
be  looked  at  without  injury  to  the  naked  eye.  At  its 
rising  and  setting  it  was  completely  obscured  by  the  dense 
haze. 

This  dry  fog  first  appeared  at  Copenhagen  on  the 
26th  of  May.  It  reached  Rochelle  on  the  6th  of  June, 
and  was  noticed,  almost  everywhere  throughout  Germany, 
France,  and  Italy,  from  the  16th  to  the  18th  of  this 
month. 

It  was  seen  at  Spydberg,  in  Norway,  on  the  22d  of 
June,  at -Stockholm  two  days  after,  and  on  the  25th  it 
appeared  at  Moscow.  In  Syria  it  was  observed  towards 
the  close  of  June,  and  on  the  1st  of  July  it  shrouded  the 
Altai  mountains. 

In  England  it  continued  from  the  23d  of  June  until  the 
20th  of  July. 

During  the  prevalence  of  this  phenomenon  the  heat  was 
intense. 

672.  In  the  summer  of  the  year  1834,  dry  fogs  were 
noticed  in  various  localities  in  Germany.  Kaemtz  ob- 
served one  on  the  29th  of  May,  enveloping  one  of  the  peaks 
of  the  Hartz  mountains. 


Give  the  instances  stated. 

12 


266          MISCELLANEOUS  PHENOMENA. 

For  three  days,  during  the  latter  part  of  this  month,  a 
haze  of  this  kind  prevailed  at  Minister,  and  the  phenom- 
enon was  seen  at  Halle,  Freiberg,  and  Altenberg,  in  Sax- 
ony, on  the  28th  and  29th  of  July. 

In  the  northern  and  western  parts  of  Germany,  as  well 
as  in  Holland,  dry  fogs  very  frequently  occur. 

673.  CAUSE.     The  origin  of  this  phenomenon  is  not  yet 
satisfactorily  explained.     Many  philosophers  suppose  it  to 
arise,  either  partially  or  wholly,  from  the  influence  of  elec 
tricity,  without  being  able  to  show  very  clearly  in  what 
manner  it  is  possible  for  this  agent  to  produce  such  an 
effect.     Others  believe  it  to  result  from  smoke  caused 
by  the  conflagration  of  forests,  the  burning  of  peat- bogs, 
and  the  eruption  of  volcanoes.     Thus  Lalande  attributed 
the  dry  fog  of  1783  to  electricity,  Cotte  to  the  union  of 
metallic  emanations  with  electricity,  while  other  philos- 
ophers traced  it  to  a  volcanic  source. 

674.  In  the  opinion  of  Kaem|z,  the  dense,  dry  fog  of 
1834  arose,  partly  from  the  combustion  of  peat,  and  partly 
from  the  unusual  number  of  extensive  fires  that  occurred 
in  this  year.     While  the  fog  was  among  the  Hartz  mount- 
ains and  in  the  vicinity  of  Orleans  and  Basle,  many  peat- 
bogs were  reduced  to  ashes,  the  fire  penetrating  deeply 
beneath    the   surface.     One    bog   in   particular,  that  of 
Dachau,  in  Bavaria,  was   burned  to  the  depth   of  more 
than  eight  feet,   the  fire  running  even  beneath  ditches 
filled  with  water.       In  July  there  were  vast  conflagra- 
tions of  forests  and  peat-bogs  in  Prussia,  Silesia,  Sweden, 
and  Russia.     The  drought,  which  then  prevailed,  favored 
the  propagation  of  these  fires  and  the  diffusion  of  the 
smoke. 

675.  The  dry  fogs,  that  occur  in  Holland,  and  in  the 
north  and  west  of  Germany,  are  attributed  by  Finki  to  the 
combustion  of  peat. 

676.  INDIAN-SUMMER  HAZE.     Throughout  the  conti- 

What  is  the  cause  of  dry  fogs  ? 
What  views  are  held  respecting  them  " 
How  is  the  dry  fog  of  1834  accounted  for  by  Kfiemtz  1 
What  is  Finki's  opinion  regarding1  the  origin,  of  the  dry  fogs  of 
Holland  and  Germany  1 


INDIAN-SUMMER    HAZZ.  267 

ncnt  of  North  America,  there  occurs,  about  the  close  of 
October  or  the  beginning  of  November,  a  warm  and 
pleasant  interval,  termed  the  Indian  summer,  which 
lasts  for  the  space  of  two  or  three  weeks,  and  agreeably 
retards  the  approach  of  winter. 

Curing  this  season  the  air  is  soft  and  bland,  and  a 
mild  temperature  prevails,  while  the  atmosphere  is  filled 
with  a  dense,  dry  haze,  that  causes  the  distant  objects 
of  the  landscape  to  appear  as  if  veiled  in  a  cloud  of 
smoke. 

677.  CAUSE.     This  obscurity  has  been  supposed   by 
some  writers  to  originate  in  the  same  way  as  aqueous 
mists  /  while  others  imagine  it  to  be  due  to  the  presence 
of  smoke,  borne  by  the  wind  from  the  distant  conflagra- 
tions of  vast  prairies  and  forests.     In  respect  to  the  first 
view  it  may  be  remarked,  that  the  Indian-summer  haze 
bears  little  resemblance  to  an  aqueous  mist.     It  does  not 
change  into  rain,  and  during  its  continuance  the  hygro- 
metric    state    of   the   atmosphere   is  different  from   that 
which  exists  when  moist  fogs  occur.     The  second  hypoth- 
esis fails,  inasmuch  as  it  assigns  a  local  cause  for  the 
solution  of  a  general  phenomenon — not  to  mention  other 
objections  which  might  justly  be  urged  against  it. 

678.  No  sufficient  explanation  of  this  singular  phenom- 
enon has  yet  been  found,  but  there  is  one  circumstance 
connected  with  it  which  may  possibly  give  a  clue  to  its 
cause. 

The  Indian  summer,  with  its  genial  warmth  and  misty 
veil,  occurs  at  that  period  of  the  year  when  the  leaves  of 
the  forest  are  falling,  and  the  vegetation  that  covers  the 
surface  of  the  earth  is  beginning  to  decay.  In  view  of 
this  fact,  the  author  was  led  to  think,  some  years  ago,  that 
the  decomposition  of  the  decaying  vegetation,  which  Liebig 


What  is  the  Indian  summer  1 

What  opinions  are  entertained  in  regard  to  its  haze  1 
Has  any  adequate  explanation  been  yet  given  ? 

What  circumstance  is  worthy  of  notice  in  connection  w'th  this  phe- 
nomenon 1 

In  view  of  this  fact,  what  has  been  supposed  ? 


268  MISCELLANEOUS  PHENOMENA. 

terms  a  slow  combustion,  (eremacausis),  might  impart  that 
peculiar  haziness  to  the  atmosphere  which  is  seen  during 
the  Indian  summer.  It  was  afterwards  ascertained  that  this- 
phenomenon  was  ascribed  to  the  same  cause  by  another  ob- 
server, Dr.  E.  B.  Haskens,  of  Clarksville,  Tenn.,  who  also 
"suggests,"  that  the  Indian-summer  haze  consists  of  carbon- 
aceous matter  or  smoke  produced  by  the  oxidation  of  the 
lifeless  vegetation.  The  warmth  of  this  season  he  attributes 
to  the  same  cause.  These  views,  however,  are  merely  spec- 
ulative. 


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...    0  25 

Latin  Lessons,  witli  Exercises  in  Parsing.     .    .    1  00 

Prepared  by  GEORGE  SPENCER,  A.M.,  as  Introd.  to 
Bullions' 

Principles  of  Latin  Grammar.  $1  50.     i 

'JSullions  &  Morris's  Latin  Lessons. 

For  beginners,  with  simple  lessons  to  be  learned  each, 
day,  and  Vocabulary,  etc., 1  00 

Bullions  &  Morris's  Neiv  Latin  Grammar,  .   ,    1  50 


2  SHELDON    &    COMPANY  S    COLLEGE    TEST-BOOKS. 

Latin  Header  (New  Edition). 

With  simple  progressive  Exercises,  references  to  Bul- 
lions's  and  Bullions  and  Morris's  Latin  Grammars,  Latin 
Idioms,  and  an  Improved  Vocabulary, $1  50 

Exercises  in  Latin  Composition. 

Adapted  to  Bullions's  Latin  Grammar, 1  50 

Key  to  Do.  (for  Teacliers  only), " .    .    0  GO 

Ctcsar's  Commentaries. 

With  Notes,  Vocabulary,  and  References  to  Bullions's  and 
Bullions  &  Morris's  Grammars, 1  50 

Cicero's  Orations. 

With  Notes  and  References  to  Bullions's,  Bullions  &  Mor- 
ris's, and  to  Andrews  &  Stoddard's  Grammars, 1  50 

Sallust; 1  50 

Latin-English  and  English-Latin  Dictionary. 

With  Synonyms,  and  other  new  features, 5  00 

First  Lessons  in  Gree7cf 1  00 

Principles  of  Greek  Grammar, 1  75 

Btillions  and  Kendriclc's  New  Greek  Grammar^    2  00 

Grcrtc  Header. 

With  Introduction  on  Greek  Idioms,  Impr.  Lex.,  etc., .    .    2  25 

Cooper's  Tirnil. 

With  valuable  English  Notes, 2  50 

Long's  Classical  Atlas. 

Containing  Fifty-two  Colored  Maps  and  Plans,  and 
forming  the  most  complete  Atlas  of  tho  Ancient  World 
ever  published.  1  vol.,  4to, 4  50 

Jsaird's  Classical  Blanual.    1  vol.,  16mo, 0  90 

tLaitsch in idt's  English-Latin  an d  Latin-EnglisJi 

Dictionary.    For  Schools.    900  pages,  16mo, ...    2  50 


SHELDON   &   COMPANY'S   COLLEGE  TEST-BOOKS. 


"  Bullions'  Analytical  and  English  Grammar  has  bean  In  constant  use  for  several 
years  in  this  and  the  other  Public  Schools  of  the  city.  It  stands  the  test  of  use. 
The  more  one  sees  of  it  the  better  it  is  liked.  I  consider  it  a  successful  work  ;  and 
1  know  that  this  opinion  is  shared  by  other  masters  in  and  out  of  the  public  service." 
»-JAMES  A.  PAGE,  Maater  of  Dwight  School. 

"  We  heartily  concur  in  the  above."— S.  W.  MASON,  Master  of  Eliot  School.  , 
D.  C.  BROWN,-  Master  of  Bowdoin  School  I 
JOSHUA.  BATES,  Master  of  Brimmer  School. 

"We  have  used  Bullions'  Analytical  English  Grammars  in  our  Public  Schools 
•early  two  years  with  success.  We  find  them  an  improvement  on  those  previously 
in  use.  Bullions'  small  Grammar  is  a  fit  introduction  to  the  large  one." — J.  D.  E, 
JONES,  Supt.  of  Schools,  Worcester,  Mass. 

"I  have  used  Bullions'  Analytical  English  Grammar  some  two  and  a  half  years, 
and  am  ready  to  give  it  my  approval.  I  have  not  failed  to  pronounce  it  the  best 
text-book  on  Grammar  whenever  I  have  had  opportunity  to  do  so.  I  now  have  a 
class  of  ninety  in  it,  and  it  boars  the  drill  of  the  school-room."— Rev.  3.  W.  GAED- 
xz&,  Principal  of  New  London  (N.  ZT.)  Institute. 


BKOCKLESBY'S  ASTEONOMIES, 

Brocklesby's  Common  School  Astronomy. 

12mo.    173  pages, $0  80 

This  book  is  a  compend  of 

JBrocklcsby's  Elements  of  Astronomy. 

By  JOHN  BROCKLESBT,  Trinity  College,  Hartford,  Conn. 
12mo.    Fully  illustrated.    321  pages, 1  75 

In  this  admirable  treatise  tlie  author  has  aimed  to  preserve  the 
great  principles  and  facts  of  the  science  in  their  integrity,  and  so  to 
arrange,  explain,  and  illustrate  them  that  they  may  be  clear  and 
intelligible  to  the  student. 

"We  take  great  pleasure  in  calling  tha  attention  of  teachers  and  students  to  this 
truly  excellent  book.  Both  the  plan  and  execution  of  the  whole  are  equally  admir- 
able. It  is  not  a  milk-and-water  compilation,  without  principles  and  without 
demonstration.  It  contains  the  elements  of  the  science  in  their  proper  integrity  and 
proportions.  Its  author  is  a  learned  man  and  a  practical  instructor,  as  the  author 
of  every  school-book  should  be.  The  style  is  a  model  for  a  text-book,  combining  In 
aViigh  degree,  perspicuity,  precision,  and  vivacity.  In  a  word,  it  is  the  very  best 
elementary  work  on  Astronomy  with  which  we  are  acquainted." — Connecticut 
Common  School  Journal. 

"  This  is  a  compact  treatise  of  320  pages,  containing  the  elements  and  most  of  th» 
Important  facts  of  the  science  clearly  presented  and  systematically  arranged.  It  is 
very  finely  illustrated.  It  is  worthy  of  a  careful  examination  by  »11  who  wish  U 
necure  tlie  best  test  -books,"— Q\io  Journal  of  Education. 


SHELDON   &    COMPANY'S    COLLEGE   TEXT-BOOKS. 


KEETEL'S  FRENCH  METHOD, 

A.  JVew»  Method  of  Learning  the  French  Language. 

By  JEAN  GUSTAVE  KEETELS,  Professor  of  French  and 
German  in  the  Brooklyn  Polytechnic  Institute.     12mo.,  .  $1  75 
i4  Key  to  the  Neiv  Method  in  French. 

By  J.  G.  KEETELS.    1  vol.    12mo, 0  60 

"  I  have  examined  Keetels'  New  Method  of  Learning  the  French  Language,  and 
find  it  admirably  adapted  for  conveying  a  thorough  knowledge  of  the  French  Ian- 
crua^ti.  It  is  an  easy  and  sure  method  of  both  writing  and  speaking  French  with 
accuracy  and  elegance."— DANIEL  LYXCH,  8.  J ,  Director  of  Studies  in  Gonsagat 
College,  Washington. 

"The 'New  Method  of  Learning  the  French  Language,' by  Professor  Keetels, 
appears  to  be  exceedingly  well  adapted  as  an  introduction  into  the  study  of  French. 
It.  is  emphatically  a  practical  book,  and  bears  the  mark  that  it  has  resulted  from  the 
author's  own  experience  in  teaching.  I  shall  take  pleasure  in  soon  giving  it  the  test 
of  a  trial  in  my  own  Institute." — OSWALD  SEIDESSTICKEIS,  Principal  of  the  Commer. 
cial  and  Classical  Institute,  Philadelphia. 

"  I  have  examined  several  books  designed  for  pupils  studying  the  French  language, 
and  among  them  Keetels'  'New  Method  of  the  French.'  The  last  work  I  consider 
superior  to  any  other  which  I  have  examined,  and  shall  use  it  in  my  classes  as  J,l»e 
best  text-book  upon  the  subject"— S.  A.  FABBAND,  Trenton,  N.  J. 

•    PEISSNER'S  GERMAN  GRAMMAR, 

A  Comparative  English- German  Grammar. 

Based  on  the  affinity  of  the  two  languages.  By  Prof. 
ELIAS  PEISSNER,  late  of  the  University  of  Munich,  and 
of  Union  College,  Schenectady.  New  edition.  316  pp., .  $1  75 

"•  Prof.  Peissner*s  German  Grammar  has  been,  from  its  first  publication,  and  is 
now,  nsed  as  a  text-book  in  this  College,  and  has  by  the  teachers  here,  as  in  many 
other  Institutions,  been  esteemed  a  superior  work  for  the  end  to  be  subserved  by  it, 
In  attaining  a  knowledge  of  the  elements  of  the  German  language.  I  cordially  rec- 
ommend it  to  the  attention  and  use  of  such  American  Academies  and  Colleges  as  are 
designed  to  give  instruction  fn  the  German  language."— L.  P.  HICKOE,  President 
Union  College,  N.  Y. 

COMSTOCK'S  SERIES, 

System  of  Natural  PJiilosojtJty. 

He-written  and  enlarged,  including  latest  discoveries. 

Fully  illustrated, ; $1  75 

Elements  of  CJiemistry. 

Re-written  1861,  and  adapted  to  the  present  state  of  the 
Science, , 1  75 


SHELDON   &    COMPANY'S    COLLEGE   TEXT-BOOKS.  5 

OLNEY'S  GEOGRAPHY, 

Olney's  Geography  and  Atlas. 

Revised  and  improved,  by  the  addition  on  the  Maps  of 
the  latest  information  and  discoveries.    New  Plates  and 
Woodcuts,   Atlas,  28  Maps.   Geography,  18mo,  304  pages,  $2  40 
These  favorite  text-books,  of  which  more  than  a  million  have  beeni 
said,  are  kept  up  to  the  times  by  the  publishers,  who  add  the  latest 
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Tuxt-Books,  so  as  to  make  them  worthy  of  the  claim  that  they  ax» 
the  best  works  for  the  study  of  Geography  now  published. 

PALMEK'S  BOOK-KEEPING, 

.Palmer's  Practical  Hook-Keeping. 

By  JOSEPH  H.  PALMER,  A.M.,  Instructor  in  New  York 
Free  Academy.    12mo.    167  pages,     ........  $1  00 

Ulanks  to  Do.  (2  numbers,  Journal  and  Ledger),  each     .    0  50 
Key  to  Do  .....    ;    .    ,    ..........    0  10 

This  work  is  adopted  by  the  Boards  of  Education  of  the  cities  of 


rk  and  Brooklyn,  where  it  is  generally  used  in  schools  and 
recommended  by  teachers.  It  is  also  recommended  by  accountants 
of  prominent  commercial  firms,  and  the  Press. 

IFJiatcly's  Elements  of  Logic. 

By  RICHARD  WHATELT,  D.D.,  Archbishop  of  Dublin. 
New  revised  edition,  with  the  Author's  last  Additions. 
Large  12mo.  484  pages,  ............  $1  75 

"This  work  (Elements  of  Logic)  has  long  been  our  text-book  here.  The  style  In 
vhich  you  have  published  this  new  edition  of  so  valuable  .1  work,  leaves  nothing  to 
be  desired  in  regard  of  elegance  and  convenience."  —  Prof.  DUNN,  BrownUnioersity. 

"  Its  merits  fire  now  too  widely  known  to  require  an  enumeration  of  them.  Tlin 
present  American  edition  of  it  is  conformed  to  the  ninth  English  edition,  which  was 
revised  by  the  author,  and  which  contains  several  improvements  on  tlio  former 
Issues."—  North  American  Review. 

IFIiately's  Elements  of  Rhetoric.  , 

Comprising  an  Analysis  of  the  Laws  of  Moral  Evidence 
and  of  Persuasion,  with  Rules  for  Argumentative  Com- 
position and  Elocution.  New  edition,  revised  by  the 
Author.  Large  12mo.  546  pages,  ........  $1  75 


C  SHELDON   i    COMPANY'S   COLLEGE   TEXT-BOOKS. 

"The  Elements  of  Rhetoric  has  become  so  much  a  standard  work  that  it  might 
Bccm  superfluous  to  speak  of  it.  In  short,  \vi:  should  not  dream  of  teaching  a  Col- 
lege  class  from  any  other  took  on  llhetoric.  Communion  -with  Wbately's  mind 
would  improve  any  mind  on  earth."1— Presbyterian  Quarterly  Hevieio. 

•     The  above  are  the  editions  formerly  published  by  JAMES  MUNROK 
&  Co.,  Boston,  and  the  best  in  the  market.    They  are  used  in  all  th 
principal  Colleges  and  Academies  in  the  United  States. 
iFitch's  Mapping  Plates.    (In  one  volume,  quarto.) 

Designed  for  Learners  in  Geography  ;  being  a  collection 
of  Plates  prepared  for  Delineating  Maps  of  the  World, 
and  Counties  forming  its  principal  subdivisions,  viz.,  1. 
The  World.  2.  United  States.  3.  North  America.  4. 
South  America.  5.  A  State.  6.  Mexico  and  Guatemala. 
7.  Great  Britain  and  Ireland.  8.  Europe.  9.  Southern 
Europe.  10.  Germany.  11.  Africa.  12,  Asia.  13.  At- 
lantic Ocean.  14.  Pacific  Ocean.  By  GEO.  W.  FITCH,  .  $0  60 

NORMAL  MATHEMATICAL  SERIES, 

iftoddard's  Juvenile  Mental  Arithmetic. 

By  JOHN  F.  STODDARD,  A.M.  For  Primary  Schools.  72pp.,  $025 
Stoddard's  American  Intellectual  Arithmetic. 

An  extended  work,  for  Schools  and  Academies.  172  pp.,  .  0  50 
Stoddard's  Rudiments  of  Arithmetic. 

This  work  presents  such  parts  of  Arithmetic  as  are  most 
useful  in  ordinary  business  computations.     192  pp.,    ...     0  50 
Stoddard's  3>w  Practical  Arithmetic. 

Embracing  methods  and  forms  of  modern  business,  with 
Analyses  and  many  varied  Examples.     192  pp.,      ....     1  00 
Stoddard's  Complete  Arithmetic. 

Being  in  one  book,  the  pages  of  the  New  Practical  Arith- 
metic in  the  order  of  that  book,  and  120  pages  of  additional 
matter,  suited  for  classes  in  High  Schools.  A  full  course  in 
one  book, "...  1  25 

Key  to  Stoddard's  Complete  Arithmetic,    ...  1  00 

Methods  of  Teaching  and  Key  to  Intcllcc.  Arith.  0  50 

SrJntylcr's  Higher  Arithmetic.    (For  Colleges),    .    .  1  25 
Stoddard  <f-  Jlcnltfc's  Elementary  Algebra. 

By  JOHN  F.  STODDARD,  A.M.,  and  Prof.  W.  D.  HEXKLE,  1  25 


SHELDON    &    COMPANY'S    COLLEGE   TEXT-BOOKS. 


JLey  to  Stoddard  &  Ilenkle's  Element.  Algebra, .  $i  25 

Stoddard  <0  Jlenfcle's  University  Algebra. 

For  High  Schools,  Academies,  'and  Colleges.    By  JOHN 
F.  STODDAKD,  A.M.,  and  Prof.  W.  D.  HEHKLE.    528  pp., .    2  00 

Key  to  Stoddard  &  Henkle's  University  Algebra.  2  00 

"  1  have  examined  Stoddard  &  Ilenkle's  University  Algebra.  It  is  a  thorough 
and  elaborate  work.  It  combines  clearness  and  simplicity  in  its  method  and  illus- 
trations, and  constitutes  a  valuable;  addition  to  the  mathematical  works  of  the  day." 
— CYKUS  NUTT,  A.M.,  Professor  ofJIat/tematica  in  the  Indiana  Anhlniry  I'nie'y. 

"I  hav»  examined  Stoddard's  American  Intellectual  Arithmetic,  and  cheerfully 
recomuiend  it  to  teachers  and  parents  as  a  valuable  elementary  work,  and  one  welt 
adapted  to  the  wants  of  pupils  in  the  first  stages  of  arithmetic.  It  is  constructed 
tipon  sound  and  practical  principles,  and  will  be  found  an  important  addition  to  tho 
text-books  now  in  use  in  our  Common  Schools." — Hon.  SAMUEL  S.  RANDALL,  Supt. 
of  New  York  City  Schools. 

"  Stoddard's  Arithmetical  Series  is  now  in  general  use  in  the  schools  of  this  county. 
They  have  stood  tho  test  for  four  years  us  tho  text-books  in  Arithmetic  in  our 
schools,  and  are  considered  by  our  teachers  superior  to  any  others  now  before  th» 
public."— J/r.  S.  A.  TOUIULL,  late  Supt.  of  Public  Schools  of  Wayne  Count//,  1'a, 


EOOKEB'S  PHYSIOLOGIES, , 

Hooker's  First  Booh  in  Physiology. 

For  Public  Schools, $0  &0 

HooJter's  Human  Physiology  and  JTi/giene. 

For  Academies  and  general  reading.    By  WORTHING- 
TON  HOOKEU,  M.D.,  Yale  College, 1  75 

These  books  are  text-books  almost  wherever  they  are  known. 
The  "  First  Book"  is  a  text-book  in  the  Public  Schools  of  Boston, 
Kew  York,  Buffalo,  and  San  Francisco. 

*•  Professor  Hooker's  work  on  Physiology  has  been  in  use  for  the  last  year  in  tho 
Informal  School  in  this  city,  and  it  gives  me  great  pleasure  to  express  my  convictions 
of  its  excellence  as  a  text-book.  In  the  course  of  my  experience  as  a  teacher,  I  hava 
used  the  books  of  various  authors  on  the  subject  of  Physiology,  but  the  work  of 
Professor  Hooker  satisfies  me  much  more  fully  than  nny  other  that  I  have  used.  It 
has  tho  double  advantage  of  being  accurately  scientific  in  its  matter  and  arrange- 
ment, and  of  being  expressed  iu  correct  and  elegant  English,  a  combination  '»f  'ho 
highest  importance,  and  yet  seldom  attained  to  the  extent  exhibited  in  this  book. 
I  know  of  no  book  for  which  I  would  be  willins  to  exchange  it."— liiciiAUU  EUWAKM. 
al  Universiti/,  Bloomington,  111. 


8  SHELDOX  &  COMPANY'S  COLLEGE  TEXT-BOOKS. 

Tlie  Elements  of  Intellectual  Philosophy. 

By  FRANCIS  WAYLAND,  i).D.    1  vol.    12mo, $1  75 

This  clearly  written  book,  from  the  pen  of  a  scholar  of  eminent 
ability,  and  who  has  had  the  largest  experience  in  the  education  of 
the  human  mind,  is  unquestionably  at  the  head  of  text-books  in 
Intellectual  Philosophy.  The  author's  practical  suggestions  on  the 
cultivation  of  the  several  faculties  of  the  mind,  aiding  the  student's 
efforts  to  discipline  and  strengthen  his  intellectual  energies,  and 
the  numerous  references  to  books  of  easy  access,-  specifying  the 
places  where  topics  treated  of  are  more  fully  discussed,  make  thia 
book  a  valuable  addition  to  the  readable  books  of  any  teacher  or 
professional  man. 

Tlie  Exhibition  Speaker  and  Gymnastic  Book. 

Containing  Farces,  Dialogues,  and  Tableaux,  with  Ex- 
ercises for  Declamation,  in  Prose  and  Verse.  Also,  a 
Treatise  on  Oratory  and  Elocution,  Hints  on  Dramatio 
Characters,  Costume,  Position  on  the  Stage,  Making  up, 
etc.,  etc.,  with  illustrations.  Carefully  compiled  and 
arranged  for  School  Exhibitions,  by  P.  A.  FITZGERALD. 
To  which  is  added  a  complete  System  of  Calisthenics  and 
Gymnastics,  with  instructions  for  Teachers  and  Pupils, 
illustrated' by  numerous  Engravings.  1  vol.  12ino,  .  .  $1  25 

Shatv's  Outlines  of  English  Literature. 

By  Tnos.  B.  SHAW,  with  a  sketch  of  American  Litera- 
ture, by  HENRY  F.  TUCKERMAN,  Esq.  1  vol.  royal  12mo,  $1  75 

"  Its  merits  I  had  not  now  for  the  first  time  to  learn.  I  have  used  it  for  two  years 
as  a  text-book,  with  the  greatest  satisfaction.  It  was  a  happy  conception,  admirably 
executed.  It  is  all  that  a  text-book  on  such  a  subject  can  or  need  be,  comprising  «. 
judicious  selection  of  materials,  easily  yet  effectively  wrought.  The  author  attempts 
just  as  much  as  he  ought  to.  and  does  well  al!  that  he  attempts  ;  and  the  best  of  tho 
book  is  the  genial  si>irit,  the  genuine  love  of  genius  and  its  works  which  thoroughly 
pervades  it,  and  makes  it  just  what  yon  want  to  put  into  a  pupil's  hands."— Prof. 
J.  II.  RAYMOND,  University  of  fiochestet: 

"  Of  'Shaw's  English  Literature'  I  can  hardly  say  too  much  in  praise.  I  hope  its 
adoption  and  use  as  a  text-book  will  correspond  to  its  great  merits."— Prof.  J.  (i 
PicKAiiu,  III.  College. 


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